You can't simply opt out of a leaderboard.
When video games give you the option of choosing between playing for fun and playing for a spot on the leaderboard, almost nobody chooses to play for fun and the entire mode becomes irrelevant.
And at least half of the people who get on the top of the leaderboard end up being people who hacked the game or people who just played against their friends a thousand times and boosted their stats.
Forza 4 is a leaderboard I've opted out of since I got the game. If I want competition, I go online.
1) They limit what the player can do (no customization, racing modifications, in-car view, etc.)
Well, compare to the case where they don't exist, did you lose anything? Also they can be customized. They can be painted and have wings added as the premiums do. They lack RM, but honestly so does most of the premium line up.
For GT6 it seems like the differences here will be completely gone.
2) They create a functional and aesthetic divide that fundamentally breaks the realistic ideal Polyphony is going for.
The aesthetic divide is there, though I hardly notice. What is the functional divide? The standard cars run on the same physics (though perhaps their perameters were in many cases lazily carried over from GT4) and as mentioned above, you can customize them.
3) They come at the expense of other features that would be more desirable in a modern, next-gen racing simulator (some of which machschnel mentioned, though there are of course many more). Time spent doing one thing is time not spent doing something else, it's that simple.
More desirable is an opinion. Also who spends time doing what is the question, as is what time they're spending. Example, premium modelers are in the process of modeling Porsches. Then a license issue comes up with Porsche and they need to stop. Should they do nothing, or maybe work on standard cars? They can't help with physics programming or anything like that since they're modelers.
Lastly, as has been said numerous times, you must use standard cars to compete in GT Mode.
This is a design decision by PD, not a consequence of standards. I also kind of don't really get it since 100% is nothing but a platinum trophy. If the standards didn't exist, GT5 could have just been GT5 without the standard car events (you could argue that PD could have added more events, but that's true even now, with the standards). You can easily replicate this.
In any case, if this is the main issue, I would understand a desire to have the option to play GT Mode without standards. But this point doesn't require the removal of the cars.
There's no debating that. Whether or not I want to 100% it or not, I will, at some point, have to use them.
Well if you want 100% yes. If not, I don't see why you would have to bother with standard cars, but I don't remember GT Mode very well.
And even if I choose to drive premium cars instead, standards are still present in my races and in the game data, so points 2 and 3 still apply.
Being in the game data doesn't matter, you'll never notice.