I use the “Clippings” addon in Firefox and it works great for me.
When the forum was vBulletin we had “canned replies” for the Infraction page.
It worked OK, but it got a bit “messy” over time.
Each of us (moderators) had different writing styles and different ideas of what points should be covered in the various “common” messages.
Over time multiple versions of messages were available.
This in itself was not a bad thing. Those that contributed had the best intentions. Not only were they making their task easier but they also helped others make there task easier as well.
The problem I had was similar to what happens when many can create Tags. There got to be so many of them that they began to cover the same ground and it became difficult to find any one in particular
I’m not saying such a feature here would be a bad idea. Only that some thought should be given to who can create them and who can use them