I worry tags will encourage laziness and on the long term not very
useful summaries for poor wiki patrollers. There are other ways to
educate e.g. help icon that elaborates or "see some examples".
Jon: don't worry about that. We already use mass revision tagging effectively this way with AbuseFilter, and in extensions like GettingStarted, mobile, or VisualEditor. Patrollers see thousands and thousands of edits, and these tags really do help with context.
I think it's good to do some exploratory design with canned edit summaries, if we keep in mind that this is new and we've never put something like this in production before. It may or may not work, but it doesn't hurt to take a stab at it. In my mind, the challenge here is not the UI. Whether it's tags or a dropdown or whatever we can work out easily. The hard part is figuring out what edit summaries are so common that they should be canned. Since there are so many different kinds of edits, that's the difficult part.