In these cases doesn't the value in "Wow, this is really useful." completely outweigh the cost of having to learn something else if it does go away? It's not like you have a finite set of things you can learn and then that's it, brain's full.
Just use it until it stops being awesome and then use something else. Life's too short and your time is too valuable to pass over things like this, where your gut feeling is probably correct.
Then, I'll have to relearn all of the traditional methods that aren't going anywhere. Normally, I'm not a uptight about this stuff, but it's the same reason that I can't bring myself to switch from JavaScript to CoffeeScript despite the fact that I _really_ like the syntax.
It's probably TextMate's fault as well as Apple's—the latter of which seems to get a perverse kick out of jettisoning things people rely on—but, I am increasingly leery of anything that has even the most remote chance of leaving me high and dry.