Better an active body, than a wasted roster spot. The Warriors also shaved 22M off their luxury tax bill which in itself is amazing. Front office making great moves. Thompson is locked in for a reasonable 6M and change next year in the event Speights walks. Worst case is the Warriors have an option on him they can decline (Think it comes with a 2.5M price tag though).
In the grand scheme of things Jason Thompson's production won't be much less than David Lee's in terms of their role. Also Thompson is a decent defender and definitely an upgrade over Lee.
He'll challenge Speights for minutes at the backup 4/5.
I think Ezeli's minutes are pretty locked in. There will be games he doesn't play based on match ups, but those weren't going to him anyways.