This trade was 90% related to the Raptors' future, and 10% related to the present.
During his short tenure with Toronto, Jermaine O'Neal played well. Unfortunately, the entire team didn't. If the Raptors want to make a serious run for the playoffs, they needed to make a sacrifice in order to make an improvement. JO put up great numbers, but he was expendable. This was proven by Andrea Bargnani, who showed that he could be just as productive as JO at the centre position.
More importantly, JO's salary ate up a lot of cap space. This trade frees up a large chunk of money for the Raptors, which will allow them to (hopefully) be buyers rather than sellers. The extra finance is also important for (again...hopefully) resigning Chris Bosh, who will have many enticing offers come his way in 2010.
Shawn Marion gives the Raptors a legitamate small forward. However, the length of his stay in Toronto remains to be seen. Marcus Banks is nothing special. He will probably get a few minutes and won't be back next year.
As for Jamario Moon - it's sad to see him go. But in all fairness, he was popular with the fans, but not that great on the court. With all due respect to Moon, he was simply a one-hit wonder.