What you say about the bad birds enlarging the hole is indeed true, they will, and they do
The thing about the perch though, that's one of those some say yes, some say no
I understand the no, it makes perfect sense, the yes though is based upon the fact it helps them actually make their nest
The Wren uses sticks to make their nest, last year Mom and I watched one little guy for like 5-minites
He sat on the perch with a big long stick, and worked his little feathers off trying to cram that thing in the house
Without the perch, no way could he have hovered long enough to get that stick in there
Six of one, half dozen of another I guess...

 

Well, seeing all these little birdhouses realllllllly makes me want to put some in. I've been reading on how to keep snakes out and apparently a baffle that is normally used to keep squirrels off feeders will help as will mounting the birdhouse on a metal pole. Guess I'll need to do more thinking.