I would adore to see bikes in GT5.
Also, I think it would be very easy to implement them in GT5 and allow everybody who wants to experience them - if they like.
Gran Turismo 5 is built like a "cluster", thus allows for easy and flawless integration of any content - as we witnessed through the past. Since release we seen updates in GUI, menus, all type of content (cars, tracks, parts), items, online and offline features, physics, technical implementations... you name it. From what we know, every single part of the code can be updated separately and can be made to exist in both online and offline.
What does it have to do with Tourist Trophy being implemented in GT5? Everything.
At this point, GT5 is already in the consoles of 6 million people, with hundreds of thousands playing online on regular basis (as we can witness from ranking-spots in time-trials when you achieve very slow times). Also, recent introduction of the DLC content and displayed ability to update not just content itself (as number on R35 TC was updated for instance), but also the policy of managing the content (with Kazunori's announcement of December release of the "Family Upgrade" feature) is clear showcase that Polyphony built an unique "platform" that can be updated anytime and server-based.
Also, GT5 is currently available worldwide as a separate Platinum/Greatest Hits release with reduced price. Now imagine this scenario:
> Tourist Trophy DLC that integrates into GT5 as a full new "*game*, available from the starting screen as a separate icon next to "GT Life". It brings everything we expect from TT game: A-Spec, B-Spec, Career, Licenses, Special Races, Dealerships, Items, Practice Modes, PhotoModes, GT Bike Shop, online Seasonal Events and everything else by the rules we already saw in the original TT game
> All bikes for purchasers of that DLC can be used in Arcade mode/Online for both bike-only or mixed races. If you and your friends wants to race bikes-vs-cars, you can do it at your consideration in the Private Lounges or in specially created Open Race lobbies
With some "Prologue-type" of price for such DLC and easy integration in GT5 I can safely presume how great amount of TTDLC would be sold in the very first dates, while community would do the rest.
They could opt for a standalone release in small quantities of course, or even release it as some future GT5 Spec 3 game with Tourist Trophy included on disc. BUt I would really love to see Tourist Trophy game integrated into existing GT5 just because it would allow to play both universes simultaneously, without need to change discs and full-integration of both offline, and especially online/multiplayer features, would create a really unique driving universe in whole history of the genre.
I really hope it will happen in the (near) future.
EDIT: Also, the very poll above is made to be persuasive and it is clearly made to create a biased-results. Options are biased to achieving a dedicated result of "no" in order to backup personal feel of the author of the poll, but not the acceptable choices. Which is really not OK from perspective of anyone who thinks different than poll-author.