Rarely do any make a huge impact in the majors. There's a reason they've qualified for the rule 5 draft, but many can make some contribution. All of the "Major League Phase" players are required to be on the 40 man roster, or they get sent back to their original teams with salary. You pick up guys you need to fill bench and bullpen holes, but that's not to say you can't find a guy that needed a change of scenery or to get hooked up with the right coach who can further their development. (i.e. Roberto Clemente).
To be honest, there aren't many that I've seen play. Randy Wells seemed to do better when they moved him to the pen, he could do well in Toronto if he continues to improve. I remember there being buzz around Wesley Wright as well. Plus, Wright's a lefty. There's always room in the pen for a lefty.
After looking it up, you're right. It is the 25 man roster.
"To prevent excessive turnover in the minor league levels, each draftee costs $50,000. If the draftee does not stay on the drafting team's 25-man (major league) roster all season, the player must be offered back to his original team at half-price. Organizations may also draft players from AA or lower to play on their AAA affiliates for $12,000, and may draft players from A teams or lower to play for their AA affiliates for $4,000."