This has been asked before but I feel the original question was more focussed on Stack Overflow than the Stack Exchange network at large.
Since there are now a lot of sites in the network and many of them use the tag system for categorising questions based on the title of something (for example, Arqade (Gaming), Movies & TV, Sci Fi & Fantasy, etc) I feel that the required knowledge of the subject matter the tag is representing is less of an issue than it perhaps is on other Stack Exchange sites.
In the below screenshot, for example, I shouldn't really have needed a score of 5 in either of the tags to perform what is essentially a cleanup process. I could of course edit all of the posts to delete one tag, so why can't I suggest it as a synonym?
This could be implemented as a per site setting so that it can only be enabled on certain sites if need be.
Can trusted users be trusted to suggest tag synonyms without a score of 5 in a tag?