You DO realise Imagins are the enemies, and Ryoutarou having to "learn" to cooperate with the enemy isn't exactly going to be easy right? You don't jump to friends in one night.
Ryoutarou himself said that his body gets tired when Urataros uses it, Momotaros is too rough on it, Kintaros leaves him depleted of energy, and Ryuutaros doesn't even let him stay conscious when he's in control.
Ryoutarou wanted to become stronger, but we haven't seen that, not even one bit. In Liner Form, he's still using the Imagin's powers. He has no simple fighting power of his own, despite what the Imagins can do. Which is what made him stay a lousy character, because at a crucial moment of the series, instead of letting Ryoutarou grow into his own man, they let him stay the wimp he is. Even Koutarou is surprised when he first meets Ryoutarou. Koutarou can fight all by himself, at a younger age than Ryoutarou, and utilises his Imagin perfectly.
Point is, the series sets itself up, with Ryoutarou training after every few arcs, drinking and eating all healthy food that his sister makes, but he never seems to be able to build any muscle, nor still do anything useful with a broom without someone being there to make sure he tied his shoelaces.
You can try to justify it by saying it's not about the outer strength, but the inner strength, or the courage to stand up after every time you get beaten down, but if you've been training, I think you'd be at least able to do something.
