Well, I will play devil's advocate on this for fun.
1 - maybe you can't qualify for a conventional loan (down pmt?) or you are getting a lower rate on the ARM initially
2 - if the adjustments are limited to maybe 2% max over 5 or 7 years, so you have a worst case ceiling rate known that wasn't terrible
3 - you want to move quickly because of market rising, or just personal timing
Yeah, that's pretty tortured logic, and I agree with you its not a good idea. But I could see in a FEW cases it might not be financial malpractice. Very few.
I just wish I could lock in a 20 year loan on our business property loan right now... Commercial is almost always a 5 year balloon to protect the bank from rate risk. But I'd love to lock in 20 years at like 6-7%. I'm afraid when we we re-up it might be 10% by then.