Man, it would suck if the hydrometer bumped the side of the carboy and broke!
I don't rack my wines/meads/ciders to secondary until they are 1.010 or less, so the SG doesn't matter anyway. They'll finish and clear, and then I know they are done. I rack when the lees are 1/4" thick- I could check the SG then if in doubt. But I never am.
If it works for you, that's great. But I think it's overall a bad idea. Krausen can stick to them, making them hard to read. They could break in there, ruining an entire batch. And since I have something like 12 carboys, I'd need 13 of them! No way.