1) Level playing field
2) Representation of the synth
The "no commercial plugin" restriction is in play in an attempt to address #1: people who only have a DAW can all have essentially the same FX to use.
The lack of modulation plugins and other rules to prevent transforming the sound significantly are to address #2.
I am against all these rules.
From my perspective, it makes entry more difficult since you can't use your paid plugins that you might know very well. I think we've proven that the free plugins are pretty well on par, if used well. The commercial ones do offer a faster workflow often, but not enough that Also, I'm all for making the best track possible at the expense of faithfulness to the synth because I'd rather hear good music.
But, this being the one-synth-challenge, and having had multiple votes over the past years on the subject, the consensus is to keep it how it is: as even a playing field as possible, showcase the synth as much as possible.
