formal rush?

Only the top houses are full and don't really give bids at formal rush. If they do, then it's to specific out of state guys they've been reserving the spots for.

Middle tier houses still use formal rush to finish their pledge classes and will usually give 10-15 more bids.

Bottom tier houses rely on formal rush to get some of their numbers, so getting a bid at these places is guaranteed.

So if you're not gunning for the top fraternities, then it'll be worth your time.

Guy above is correct. I just want to add that you will still have a great time at whatever fraternity you get as long as you feel like you fit in with the guys. Some people go in far too caught up in tiers and either get nothing, or go somewhere they don't completely like. Definitely go to whatever fraternities you can the first night to get a better feel. Narrow down your options some and go back to the ones you liked the most. Good luck