When I originally suggested the synonym, it was a combination of pragmatism and misunderstanding of how synonyms worked.
At the time, Railo was still generally considered an "alternative" to CF -- rather than a solution in its own right, which it is now, (or at least is tending towards) -- and I don't know that it was possible to easily monitor multiple tags, so it made sense to have the already most common "coldfusion" tag as the main one, since that's what most people used.
I also thought the tags would simply pointed/aliased to each other rather than be replaced, so the original tag would still be visible.
Given the way tags actually work, that it is easy to combine tags in search/bookmarks, and that Railo is more and more its own thing, I think I'm in favour of removing the synonym, so cfml can be a standalone engine-agnostic/parent/overall tag.
There's still a part of me saying that the masses will still just use/look at [coldfusion], and this would simply be extra work for those of us that monitor/fix CFML questions, but in general I prefer doing things correctly and educating as necessary, rather than dumbing down for the lowest common denominator.
A quick note for the uninitiated, a rough comparison is:
C# <-> CFML
Microsoft .NET <-> Adobe ColdFusion
Mono <-> Railo
It's not a perfect analogy, but hopefully gives an idea...