The presumption is flawed.
In general, all litigants are treated equally, irrespective of whether they are represented or not. The court will give appropriate latitude in applying process and procedure to all litigants with the expectation that trained solicitors/barristers will need less of it. They will not penalise a litigant because their representative has made a cock-up, but they will make their displeasure known to the representative. In general, the higher the level of the court in the hierarchy, the less latitude they will give: small claims courts are very forgiving to everyone, and high/supreme courts have an equally low level of tolerance to everyone - if you don't bring your A-game here, don't bother playing.
Whatever the level, there comes a point where latitude towards one party becomes prejudice against the other. If one player forgets their boots and shin-pads, how long should the other party be expected to wait while they go home and get them? If I've turned up ready to play, why should I be screwed around in time and money because you didn't?
As a practising arbitrator, I have had to decide, for example, whether a party can amend their claim because ... they got it wrong. Sometimes the other party is fine with the amendment (I live for these rare moments), but often they oppose them on the not unreasonable ground that they have spent a considerable amount of time and money preparing a defence against claim A and now they will need more time and more money to defend against claim B. Justice delayed is justice denied. And what about claims C, D, and E? So how should I rule? Don't answer - rhetorical question; each case has to be decided on its individual merits.
The point is there is no amateur league when it comes to litigation. Just like there's no amateur league when it comes to brain surgery or bridge design. When you sue someone or get sued by someone, know what you're doing or hire someone who does.