Think about it from the companies point of view. Hybrids are additional information which has not been counted for.
You start getting hundreds of thousands of hybrids taking up memory space it will have negative effects.
My opinion comes from seeing what happens to Blizzard servers when "hacked/duped/etc" items are added.
Not only do they make for unfair game play, they are bytes of information not counted for.
The information is stored on my PS3, not some central database. And anything I adjust on the car through hybriding, is already either a part in the game or an adjustable game parameter, it has no effect on any memory, anywhere.
Leaving aside the clowns that overdo their cars and try to cheat online or in TT's, hybriding is the greatest thing ever in any GT series. Let me give you an example of why that is.
I've always been mystified by FF's and the FTO in particular. It should be blazes fast. It's light, has a beautiful, high revving, peaky power band. Wide body, shorter wheelbase, car of the year in Japan in 1994!! But it sucks on the track because it won't turn.
Enter hybriding and 15 downforce on the front and voila, instant supercar!!! I lost 30 hp in the process, same as you would lose in any car with 15 rear downforce, but the effect is dramatic. Gone is entry understeer, and you can now power on a little earlier, at the cost of a little top speed and acceleration. The effect is dramatic. It's actually fun to drive now and with a little ballast on the rear, you can actually drive it like an MR if you want to. It's now competitive at 450PP on SH tires, able to do mid 7:50's online, right in the mix of many good cars.
I think I can now make many FF's competitive with this one hybriding tool so I would ask, how is that bad? Line up at any race at the Nurb and you see at most 10-15 cars you know are really competitive. What if there were 50 or 100? This is possible through hybriding and without gaining an unfair advantage, just competitiveness. The PP system is intact because the added downforce costs you HP same as adding a rear wing which no one would call cheating.
Hybriding and all it entails, would be the greatest thing to happen to the GT series ever, no question about it.