I think it's a good idea, as long as they can't race against cars. We can't make a "Karts vs Volkswagen Bus vs Ferrari F1" events for a reason, after all. It would feel too arcadey and amateur, not to mention silly.
I don't like bikes at all, but with PD's Midas touch and the "vehicle museum" feeling within their games, and the historical mentions and their importance, I think I could learn to appreciate bikes a lot more, at least virtually.
I personally would find it interesting if they were included. I'd also like to see more trucks included as well (like more racing or rally trucks). They could probably put Craftsman Trucks in to complement Nascar. Bikes would have to be restricted to a special event I'd think though like Formula One cars should be. Maybe include a special seasonal for Go Karts, Formula One, Bikes, etc. every now and then so people have reason to own them.
Agreed, I want more Pick-ups/Trucks in GT (like the Tacoma X-Runner), they are cool and beautiful, and Cargo Trucks (just the cabins, of course). Also the professional Super Truck series, those are heavy and fast. XD It would rock.
I think Toyota in particular has a lot of beautiful Trucks that are not yet featured in the game, which is a shame.
I would love if GT had both vintage and modern F1 cars (and not **just** Ferrari), but, that's a licensing issue, unfortunately.
In comparison, though, it already has a lot of vehicles for just a few tracks. I hope they rectify this department soon.
. If I want to race bikes I'll buy a game about bikes. Tell you what though why don't we throw lawn mowers in the mix too and horses and maybe a spot of camel racing for good measure, after it's just a game. 
