Good trade for teams.
The Spurs did a wonderful job at adding another role player to fit around the big 3. Jefferson may not be what he used to be... but he can still hit open shots and play some defense. I don't see how anyone can really argue this, the Spurs can afford him and they want to win a title right now while they still have all their core players together. I applaud them for going for it, frankly I didn't consider them a top team going into next season unless they added some depth and talent. With this trade that is exactly what they did.
With the Bucks, its simple this is for salary reasons. They needed to shed some salary to afford signing there young players (Charlie V and Sessions). Richard Jefferson really wasn't a fit in Milwaukee anyways. He just settles for too many mid range jump shots, he really isn't the type of player who can take over games and be that franchise player for the Bucks.
He was rumored to go at the trade deadline, no one should really be caught off guard with this one.