From the Sona book linked to from your website (2nd link, the first is broken, also thanks for all that I'll take a serious look at Sona):
"What might a Chinese critic have replied [re learning Esperanto]?'I'd rather learn Latin and have done with it'"
There's the reason simplicity is so important! (original topic of the thread) 
I guess you could call it marginal utility.
An auxlang has to be much "cheaper" than a natural language.  If I'm going to spend time learning Interlingua, which few people yet speak, it had better be a whole lot easier to learn than Spanish, which might be a more useful way to spend my time even though it is a harder language. 
And that's why I was thinking speed-of-learning tests might have value, actual data so purveyors of auxlangs can actually make claims about relative acquisition rates.
On 03/20/17, Paul Bartlett<[log in to unmask]> wrote:
On 2017-03-20, Robert South wrote (excerpt):

> Also finiteness. I think this is a vitally important selling point. It
> must be possible to learn the whole language. There should be an end
> state where nobody who has studied the language longer, or been more
> engaged with developments, will be able to suprirse you with stuff you
> don't know. And that's why the language should be oligosynethicish, and
> the set of allowed sounds should equal the set of root words. Which is
> why it must be largely apriori, though it can follow natural language
> where it can (why not?).

> That way a person who has just learned all the basic elements will know
> all the words that might be used by a lifelong speaker, kinda
> theoretically. [...] Oligosynthetic languages would have it
> too, many different ways to combine the basic vocabulary to express the
> same idea, but all such expressions would be made of the same elements.
> Being oligosynthetic is itself enough to make
> an artificial language radically different, while also improving
> finiteness, though it adds some to complexity (but on a scale that pales
> in comparison with the learning of vocabulary).

If we are going to consider oligosynthetic auxiliary languages, I
suppose that Searight's Sona would be about as good a place to start as
any, although one shortcoming is that materials are mostly available in
English. I have numerous materials on Sona (including a printable scan
of "The Book") in my personal webspace at
also available for mobile devices as .

Paul Bartlett