I do a number of sealer layers as well, but I usually use gloss. The idea is to let washes and inks flow into crevices better, as well as to act as a "save point" -- if I slip, or wash runs where I don't want it, I can wipe it off with a finger and/or clean wet brush. The problem with gloss is that it can make it more difficult to see highlights -- as sounguru said, you have to sort out what's a reflection of your lighting and what's actually a highlight by moving the mini around more as you paint. However, I use an LED arc so I don't have point sources, making this less of an issue.
I don't do as many layers as sounguru, though -- I hot-glue my minis to jar lids so I have a handle (I still end up touching them, but at least I'm not grabbing them). Prime, base coat, gloss. Then if there are any other airbrushed colors I gloss after each of those, especially if I'm using tape or Silly Putty as masking for camo. After that, the only seal er I do is during the decal stage, and at the end (where I do gloss then matte, so that I can tell if the coating is being worn away).