Matt is an asset to the team. He actually is an adequate team defender and rebounder. He knows the system and doesn't make many mistakes in rotations or play executions. He doesn't turn the ball over. He doesn't try to do more than he can, he shares the ball if his shot isn't there. And on offense he has to be accounted for by the other team's defense, which makes them work. He will pop the three on a kickout if his man drops off to challenge a dribble drive, or if he's closed out on he is increasingly able to ball fake and drive for a floater or a baby hook, and his man knows this, so he won't challenge. This really helps to open up the lanes. And, he's completely over himself, in fact he always was, which is really important to be a Spur. There's a reason the Spurs are paying him. They are actually smarter than you about this.
He's really not comparable to Tiago, two very different sets of capabilities they bring when they're out there.
Compared to Novak, Bonner is very superior. Novak is nowhere near an adequate defender or rebounder, and his 3 point shooting is somewhat better than Bonner's (he's able to get his shot off quicker for example), but not THAT much better.