Just a couple thoughts from my own experience:
First, I'd recommend getting a blowoff tube that's the same diameter as the opening of the carboy, so you don't have to mess with a stopper, and so the blowoff has more room. Your little blowoff tube could get clogged with foam as easily as an airlock. I did a high-gravity beer once where the krausen pushed halfway into the blowoff tube. Very messy.
Second, it appears that the milk jug that the tube goes to is capped. If you do this, drill holes in the top of the jug (not the cap) so the CO2 can vent out of the jug. Otherwise, the pressure could build up and rupture your jug. Again, messy.
Feel free to do what you will, but in my humble opinion, a couple extra steps will save you more cleanup in the future.