even roger was taken down, in the end.
and it doesn't matter how strong he is, if he's got a collar.
as to how did he get the collar. somebody probably tricked him, or ganged up on him (3 admiral, Garp and Daibuddha. for example).

besides, he has already been to the final island, and knows what 'one piece' is (my theory - the final tablet of the truth), what need does he have for a crew.