Imperial Legion Not Joinable, It's Confirmed Options

[Wurlon]

It's been confirmed the legion will not be joinable as a faction, which I think is kind of right considering the empire becomes overrun by daedra. It would be funny to go into a wrecked tower, talk to a leader and he tells you to go rescue a slave while daedra are attacking outside....

Sad isn't it? Oblivion is truly going to be quality instead of quantity this time ....

[DoomedOne]

Imperial Legion is a large part of the main quest, and they can't be a joinable faction for a nhumber of reasons:

You'll be dealing with some high-ranking members for main quests, possibly the dragon, but if it were a jojnable faction you could end up as the dragon, which wouldn't make sense.

At one point its incredibly likely you will be leading the legions into battle, or the heir. If you started the in the legion as and never went very far then it'd be strange for them to take orders from a low-ranked legionaire.

It will probably be part of the main quest to join the Blades, probably one of the means of beating the game (remember he said many roads to finish the game but you end up at the same place). It sort of defeats the purpose of being a Blade if you're a Legionaire.