The talk about source languages (or their speakers) drawing an auxlang into irregularities brings me back to this topic, though I should explain clearly what I am and am not talking about.

A basic vocabulary does not mean that neologism is disallowed; it is basic precisely because an advanced level exists. Rather, a well-defined basic vocabulary means that learners and teachers can both know that certain words must be learned to function adequately in the language. There are other words, but they may be learned passively and/or later. Active mastery of the basic words is what you should concentrate on, and the fact that there are relatively few of them--about five hundred or so that are truly basic and perhaps twice as many (inclusive) that are considered generally necessary--means that they will occur often enough in an introductory text to hammer them into the learner's memory.

Limiting the number of basic words also means that the meanings will diverge from those of the source language(s), because working with that few words will require divesting them of the nuances (or picky precision) they have in the source languages. This will both improve the neutrality of the auxlang and (through repetition of the form in contexts where the source word would not be used) serve notice of the auxlang's autonomy.

Also, since everyone beyond beginner level would know the same basic words, the temptation to introduce unnecessary neologisms should be reduced: only common and lengthy expressions would be all that liable to replacement, and a well-designed lexicon should limit the need.

Of course, it's tedious to keep running into the same words over and over in a text, though a vocabulary of about a thousand words should reduce the problem and the presence of occasional borrowed words, mostly names, would also break up the monotony. I suspect this is another reason why Hogben had synonyms in Interglossa: it allowed a little variation in a longer text.

However, technical items would contain borrowed jargon--a well-designed auxlang should have rules for external derivation of such terms. Similarly, something literary would include literary words not found in basic texts. So lexical boredom shouldn't be a large problem. But the added vocabulary could be mostly passive for a lot of users, and it would be mostly limited to a specific field, again avoiding an increased learning burden for beginners.