Each ability has a drawback that makes the ability somewhat unpractical to use. In Yuu's case, his ability allows him to possess people, but only for a few seconds, and his body becomes lifeless while he is doing so. However, later in the show, we discover that his true ability is "Plunder", allowing him to actually steal other abilities, making him the most powerful human being on the planet with a rather minor drawback, since he can use all the abilities he steals without any apparent drawbacks. In the final episode, his overuse of his Plunder ability makes him forget everything about himself and eat away at his sanity. Thus the true drawback of his Plunder ability isn't to make him defenseless: it is to overwrite parts of his brain with the powers he steals.
One of the only things that is a major hindrance to Yuu's ability is that he must see his target. Makes a lot more sense why the author choose his first encounter with other powers (Tomori and Takajou) to be people that could completely dodge his ability. Takajou was entirely too fast for him to clearly see, and Tomori turned invisible when he finally was mad enough to do something, which allowed both of them to confront him without fear of his possession or later on, his plunder ability.
Over the course of the series, Yuu undergoes Character Development and becomes a better person, starting the series as someone who'd use his power to cheat on tests, and ending the series as someone who's willing to track down and absorb every ability in the world, no matter what it costs him. Now imagine what would happen if Yuu discovered his plunder ability without changing for the better, or what would happen if a person with no conscience or regard for the well-being of others got it.
Yusa was never able to meet Misa after she died because Misa had to use Yusa's body to be channeled. Instead of disappearing for good when Yuu takes Yusa's power of channeling, Misa should have had a chance to meet Yusa at least one time by having Yuu channel her.
On the other hand, Misa made the analogy that Yusa was holding on to Misa's spirit the same way she'd hold a balloon, and that if she lets go, the balloon would float away. To use the analogy, even if Yuu had Yusa's ability, he might not have been able to grab the "balloon" before it floated away (thus meaning that Misa's spirit departed before Yuu took her ability)