The only downside I see (after a couple of bat walks ) is that once they roost and have been seen to roost in your bat box, the bat box is then protected under law and cannot be disturbed. Obviously not a major issue unless you place it on your house & then want to extend the house (or the next buyer does)..
As for Camo's point, bats apparently tend to stick to the eve's of houses and any you find inside the loft are usually lost and are trying to get out (usually die unfortunately), unless of course their loft is full of holes. They can be moved, but it can't be done during particular seasons got good reason (mating/rearing young) & you will need to consult bat experts (que lots of whiney house owners who aren't able to extend there house thats been a bat roost for hundreds of years).
They're a protected species for good reason, everything tries to kill and eat them (bird/squirrels!), yet do bugger all damage to property and are amazing to see.
We usually get loads in our garden but have seen none this year, last years very wet conditions, meant that there wasn't that much in the way of insects and even then, bats cant fly (well?) in the rain. So the expert on our last walk was very concerned about the UK population
Get a bat box built, just put it in a tree would be my advise.