I have no idea if they are any good for mites, or for ventilation. If they do, then great. I don't really care about all that.
I love the fact that I can pull out the insert and get a really good idea of what is going on in the hive.
Things that I can see on the insert:
-Mite count (currently low)
-What they are doing with the comb: lots of chewed capping debris, lots of crystallized honey where they are cleaning out the comb.
-When they are building - new wax scales that drop...there were a couple, they are ramping up production
-SmallHiveBeetle - in the fall there are usually dead ones, and then in the spring as the bees clean the dead ones out of the comb that froze. In fact, last night one of my inserts had a few little evil larvae wriggling around. I'll have to check the hive today to make sure they aren't in the hive.
-Wax moth larvae - not so useful, but I can see the little piles of caterpillar poop where somewhere in the hive there is a moth larvae working. I've never found the caterpillar in the hive, though.
-Pollen - they usually drop a few pollen loads, it is neat to see what they are collecting, color, taste, etc. Sometimes they drop a load of electic blue or other exotic color pollen.
And it isn't good to let too much stuff accumulate in the trays, but the little ants try to keep it clean, and don't bother the bees much. The trays should be cleaned out on a semi-regular basis.
So even if there aren't health benefits to a SBB, there are very interesting reasons to have one.