I agree. If 7.5k is not the right magic level to bless with this privilege, how about 10k? Or 15k? Or 20k?
There has to be some point at which we can start offering this privilege. Once a user reaches 20k reputation, they attain trusted usertrusted user status, which gives them a whole lot of leeway already. For example, they have full and unrestricted editing privileges for the tag wikis.
It's quite frustrating that users who are already trusted with other important privileges cannot also propose tag synonyms, considering that the system already requires that their proposal be voted upon and agreed with by at least 4 other users who have attained the requisite privileges. I can't see anything bad that can happen by giving high-rep users the right to make such suggestions.
As to why this is important: I've run into a lot of lesser-used tags that have what I consider to be some pretty obvious synonyms. The score of 5 that is required to suggest tag synonyms seems trivial, but only when you consider common and frequently-used tags. It can be very difficult to attain a score of 5 in some of the more obscure tags.