But it's not like the old Riders are going to be in every single episode; probably a four episode run at best, and maybe a team-up toward the finale. Plenty of these guys have put in guest appearances in other tokusatsu (hell, there was a period where you couldn't find a tokusatsu without an appearance by Ryouhei), and a few of these guys remain friends with directors, writers, and/or producers and sometimes will work as a favor. Like, Takamasa Suga supposedly stayed close to Ryuki's main director Ryuta Tasaki (who's Decade's main director), which is how he ended up appearing in Sh15uya (which Tasaki directed).
It's just another boneheaded move on Toei's part. I don't subscribe to the whole "it's just a toy commercial, story's icing on the cake." EVERY show is meant to sell product, but a lot of writers and directors on these shows are still dedicated to delivering quality work. Besides, it's a boneheaded move in terms of marketing. Rider DVDs are still on shelves -- what better way to stir up interest than expose a lot of these characters to viewers who missed them the first time around? Now people who didn't see the Kuuga series won't bother to check it out since it has nothing to do with what they might see and like in Decade.