While 30 starts are better than 10 starts, if the ten starts are during a push for the World Series, their value may be higher than 30 starts for a "maybe get to the playoffs" team. Plus, the receiving team also gets a full season the following year (however many games that might be). And in regards to your second coment, adding pitching at the deadline also allows a team to keep prospects in the minors. If no one is added to the roster at the deadline, prospects may have to be brought up earlier.