Short Answer
It is routine as a matter of course to deny all conclusions of law in the manner you identify unless you are willing to judicially admit them on behalf of your client, as you would if you admitted them. Except for personal jurisdiction and venue, you are very rarely willing to make such a judicial admission.
The only reason to admit conclusions of law, in most cases, would be to narrow the scope of the proceedings (and hence the cost of the litigation), for example, in a very simple case where only one or two well defined issues of fact were involved (e.g. whether or not a a defendant had made the payments when due on a promissory note, where the evidence will hinge upon evidence in the form of cancelled checks), and the plaintiff will get his attorneys' fees if he prevails while the defendant will not.
Long Answer
A paragraph by paragraph response of the type you describe is commonly found in an "answer" to a "complaint", a "reply" to a "counterclaim", or a "response" to a "petition", each of which initiates a new claim for relief in a lawsuit. Sometimes, a similar response is given to a "motion" when it is functionally equivalent to a "complaint", a "counterclaim" or a "petition" in the sense that it raises a new claim for relief. The form of the response reflects the fact that it is being responded to in the same form that one would respond to a "complaint" in an "answer" is following those rules of pleading. These responses have to be made typically within three to five weeks after receipt of the document making the claim.
As a general rule, a complaint sets forth a "short and plain" statement of the circumstances that give rise to a claim for relief in favor of the person filing it, against the person(s) from whom relief is sought.
Prior to the adoption of the Federal Rules of Civil Procedure, under what was known as "Code Pleading" (after the widely emulated "Field Code") that governed pleading in some states, you were required to allege all facts you intended to offer in support of your claim for relief in your complaint in itemized paragraphs that made detailed factual allegations and law was not cited at all except in the headings describing the name of your claim for relief (which usually arose at common law rather than by statute). An answer had to admit, deny, or partially admit and partially deny as explained, each allegation (in addition to offering up affirmative defenses). These admissions and denials were supposed to limit the scope of evidence admissible in trial and matters subject to discovery and/or subpoena at trial.
These rules were relaxed in the 1930s when the Federal Rules of Civil Procedure adopted a "notice pleading" standard, which was widely followed by state courts in all but a handful of states like New York and California, but were partially tightened again via case law under doctrines known by the lead cases of Twombly and Iqbal from the U.S. Supreme Court.
For example, under modern pleading, it is no longer necessary to allege every fact that will be offered at trial, but it is necessary to state specific, non-conclusory facts that make is plausible that the person asserting the claim is entitled to some relief on some legal theory (which need not be specified in the complaint, although it usually is specified).
Some special response to allegations of a complaint include a denial for lack of information and knowledge (which simply holds the plaintiff to their burden of proof on that point, but is not supposed to be made unless it is true that the defending party doesn't know if the fact is true or false); a statement that a document "speaks for itself" (when the provisions of document are paraphrased and the details of the document's language is material to the outcome); and a statement that something is a "conclusion of law".
A "conclusion of law" response isn't really a statement that it is "improper" to make a statement of law in a complaint, it is simply a statement that the law exists and may be relied upon by the court according to what it says, whether or not it is included in the complaint (and it is not required to be included in the complaint) and an unwillingness to concede an interpretation of a question of law when filing an answer. Subsequent case law has established that this part of pleading law is unchanged by Twombly and Iqbal.
Some uncontroversial conclusions of law are routinely admitted, for example, personal jurisdiction and venue (subject-matter jurisdiction may be raised at any time, even sua sponte by a court).
Other conclusions of law routinely receive the lack of admission or denial response that you identify, so that those legal questions can be argued later during the course of the litigation by motion, in a trial brief, or in closing arguments at trial. Doing so saves scarce time at the outset of a case in preparing the "answer" or other responsive pleading, and leaves a lawyer free to refine his or her legal argument for his client later on in the case, rather than conceding a legal argument at the outset.
Frequently, someone making this response does not seriously contest the accuracy of the legal proposition which is often useful to include in the complaint or other pleading asserting a claim to explain why the facts recited are relevant, but simply wants to keep their options open. Also, often a proposition of law cited is true, but incomplete.
For example, someone might quote the statute of frauds which states "a contract to answer for the debt of another must be supported by a writing signed by the party to be charged", without citing the case law applying that language in a manner that establishes exceptions to the general rule (e.g. part performance, or a judicial admission, or a statutory obligation to guarantee certain kinds of obligations) that a further investigation of the facts might reveal is applicable in your case.
If you could not respond in this way to conclusion of law, each response to a conclusion of law could require a mini-brief to appropriately partially admit and partially deny without putting one at risk of making an over broad judicial admission, which would be a waste of legal resources so early in a case when many legal issues could be mooted by later legal developments and the relevant legal issues may be vague because the facts are only known imperfectly.
