Carlos Mortenson is a perfect example of a guy who plays rags for raises with a purpose. He does it to maintain his 
image. You can never put him on anything. It's a mess, but it works when done right. The 
variance is very high. but it can have the effect of loosening your opponent up enough to bust them when you 
catch a high 
pocket pair.
I have to admit I play rags for raises often against the right opponent. Two hands in peticular happened 
back to 
back and illustrated the downfalls and benefits of doing so...
Hand 1: I was a high 
stack and was dealt 6c 5c. Guy raises 3xBB and I'
m in the pot with 2 other callers. The 
flop comes 6 6 T. Another guy in hand had A 6 and doubled up through me.
Very next hand: I was dealt 6s 7s. Guy raises to 5xBB. I am the lone caller and the 
flop comes out 6d Jh 6c. Of course he has a high pocket, and I almost double through while busting him.
The reason it's sometimes correct to 
call big raises with rags is because of 
implied odds. Big raises signify high 
pocket pair. What that means is you will get a lot of 
action when you hit your hand. In other words, your 
implied odds are almost guaranteed to be fantastic. You're going to double through more often than not when you hit a 
monster with your rags against a high pocket. You just have to know when it's time to 
fold. You have to understand that you need more than 
one pair to stay in.
Times NOT to do this:
-When blinds are sky high in a tourney
-When you're short stacked in a tourney
-In a cash game when you can't afford it
-When your opponent has a low 
stack (no 
implied odds)