Hi Julie,
In a perfect world, we would all rather stick to assignments, they are easier and cheaper. However we don't always have the option of assigning a contract. Some sellers won't allow you to do so.
If you do a simultaneous close you have to pay two sets of closing costs. One set for the first settlement and another set for the second settlement. Some of the costs can be saved because you don't have to do two title searches, your settlement company may be able to cust the costs on one of the settlements because the work has already been done for the other etc...
You don't make more money when doing simultaneous closes, instead people tend to do them when there is more money on the table to be made. You can hide your profit from your buyer when you do a simultaneous close. In an assignment YOUR PROFIT stands out like a sore thumb to them in the form of an assignment fee. If you put a home under contract for $10k and are assigning it for $15k that is all the buyer will wee when they are at the settlement table and it will usually disturb them. But if you spend an extra $1000-$1500 on a simultaneous close, they won't ever know your mark up, at least not before settlement is done.
I hope this helps.