Celticsblog posters, for time immemorial, have had an afflicting fixation on athletic pure centers that block shots.
Ainge rarely puts such players on the roster, favoring players with more well rounded skillsets.
You can trace it back to the Perkins trade, to Ainge not drafting Deandre Jordan, and probably well before that.
R-Williams is just the latest example.
The problem is that the Celts have a system on both ends built around versatility, skill, positioning, etc. RW is raw and may get there someday but he isn't really equipped to provide significant minutes at the moment.
Flashy blocks are fun but positional defense and communication are actually way more effective.