You have a number of misconceptions in your question.
Don't such unilateral termination clauses (sans notice) make a contract void and unenforceable?
The clause itself could be unenforceable if B is a consumer. For example, in england-and-wales, section 62 of the Consumer Rights Act 2015 provides:
(1) An unfair term of a consumer contract is not binding on the
(4) A term is unfair if, contrary to the requirement of good faith, it
causes a significant imbalance in the parties' rights and obligations
under the contract to the detriment of the consumer.
The Act also lists some specific scenarios which may be considered unfair, in schedule 2. If your contract has no fixed end date then the following is relevant:
- A term which has the object or effect of enabling the trader to terminate a contract of indeterminate duration without reasonable
notice except where there are serious grounds for doing so.
However, note the effect of section 67:
Where a term of a consumer contract is not binding on the consumer as
a result of this Part, the contract continues, so far as practicable,
to have effect in every other respect.
Even outside of consumer law the latter point is generally the case i.e. if a clause is unenforceable, and it can be severed from the rest of the contract, then the rest will remain in effect. Severed means that removing it doesn't render the other clause(s) meaningless or absurd.
As it stands, both parties are allowed to terminate so long as they communicate, right?
Not as a general principle, no. There are cases where a contract can be impliedly terminated without relying on an express clause (e.g. if both parties abandon the contract), but you can't just freely terminate at will if the contract doesn't allow you to (unless we are considering a specialised area of law with a statutory right of termination e.g. landlord/tenant law).
And without mentioning notice periods or acceptable causes for termination what value does a clause like this add?
A can terminate without any notice period i.e. it terminates with immediate effect.
More generally what is the utility of a termination clause when it does not add details about notice periods, or acceptable causes etc?
You don't need to have a notice period or acceptable grounds unless you are dealing with a specialised area of law where those are required by statute.
A simple termination seems a recourse that is implicitly allowed, in any case, right? Whether I put such a termination clause or not? e.g. "Both parties have a right to terminate this agreement at any time" What's the utility of such a clause? Even if I didn't write this explicitly that's what will be the default position?
No. There is no implied right to terminate. Again, unless a statute provides otherwise for the area of law.