I just don't see it like that. Plenty of people do use the slower cars in most races I see online, either because they choose to handicap themselves, or because they simply like that car best. In my experience this is pretty common, to have a favorite car and race it against most odds. With more different cars in the (e.g.) GTR class, you'd have more chances of having one of the cars really appealing to a player, consequently making for more diversity at the races, which is easily one of the few most appealing things in races.. Diversity.
And like I said in a class with the main attributes of cars restricted like in any series I can think of, there's only so much variance in performance you can get.
Suppose the cars all have pretty much the same power, weight and dimensions as the GTRs we have, then the permutations of all the different components, such as drivetrain, chassis, engine character, tires etc, will eventualy give you a very dense data cloud, if you plot the overall performance value of each car across all tracks:
This is the general picture we have with 3 GTRs:
A few tracks (A,B,C, etc) with one car (colored dot) dominating (Y axis is laptimes), others with one car left behind.
Add three more and you get a denser cloud.
If you took out the first three cars (red, green, and blue), you'd have the same sort of dispersion, with one car running away at a certain track, and being left behind the other two cars at other tracks. Maybe with some extra bias from one car being a bit more dominating over all like we probably will have till LFS settles into later versions.
But with 6 cars there's less gap between any two cars racing at a certain pace on any given track, and that also means that in a full season, there'd be a more even distribution of points. There'll be closer racing in the races than in the hotlap charts for any given track, from the same effect that a third driver can catch up to two faster drivers slowing each other down as they fight for first position. The more cars, the more the racing field will tighten, the less your chance of being left hotlapping ahead, or back, or in the middle of the pack.
And part of the reason I think 3 more GTRs would amount to more fun than 3 cars in another GTR class for 2-class racing, is that the 6 car class guarantees you'll be racing someone, whereas merely adding another 3-car class with the same dispersion in performance as we have in GTRs now (and in other classes too) will add a bit of fun from enabling 2-class races, but other than that you'll still be racing against cars in your own class that are seldom at the same pace as you (arbitrary handicaps notwithstanding).