I've started doing this too, actually... I ran out of room long ago, so was wondering how I could keep Miis around, and fell back on this.
However, I think there may be one drawback (and maybe someone can confirm/deny it)...
When you move the mii off the console, I wonder if its Wii Sports record will be lost. I've only tried it so far with people who don't have any record, or likely won't be around anytime in the near future to play again.