I just hate how Standards are treated as a whole in Gran Turismo 5. Maybe they aren't as pretty and as customizable and adaptable as Premium cars, but they are just treated like... almost Easter Eggs. Filler.
I think that basically describes the reason they were put in, to mask the relatively limited amount of Premium cars, especially regarding certain categories (mostly the humdrum kind, usually a GT specialty).
To be honest, I'm treating them like that too (apart from a few opentop cars since they received a 'decent' cockpit view) and don't even bother collecting most of them.
And even though I did object to them being included when it was announced what a Standard car actually was, I didn't foresee them spending very much additional time improving them and I think PD thought too that they 'could get away with it' as they were.
Having a default range of Premium wheels applied to all Standard cars probably wasn't that time consuming to do, but making hundreds of different silhouettes for closed cars probably was.
In hindsight they might even regret the decision to include previous gen cars in a current gen game, given the additional time spend on assets they can't use in the future (at least I hope not) under 'pressure' from user feedback they might not even expected to arrive this massive on this subject (at least I didn't as I thought their main purpose was clear).
I'd rather they'd spend all that time making more Premium content instead but it seems I'm in the minority for wishing that.