The biggest negative is that many of the kits sold require you to use the bags designed for them, and the bag is gonna cost you more than a decent new paint brush.
If you've got a good (expensive) brush it's worth cleaning it properly after each use. Or you can get one of the "brush restorer" solvents that will take just about any paint out of any dried up old brush. About $9 worth of "restorer" will do a good job on any $3 brush you've got, and might even clean two of 'em.
If you roll a good plastic bag around them, squeeze out as much air as you can, and fold the end around a couple of times, using the vacuum pump doesn't really add a lot to how long they'll stay wet enough. It just makes it a little more obvious if you poke a hole in the bag.
If you use vacuum on wet brushes, evaporation of the "wet" will eliminate most of the vacuum you created after a couple of days anyway.