That's exactly it. If you sell 100 naked MSFT puts in a $100,000 account, don't expect to survive, but if you sell 50 different naked calls and 50 different naked puts (in an IB account of course) there is not much that can happen to you and you'll make 20% or so every month.
Great, so now you expose yourself to both a rally tail and a bear run. A naked call isn't going to reduce the risk of your naked put by all that much -- you're getting twice the premium but now you're opening yourself to a bear AND bull run. What happens if I sell a lot of 60 Jan calls on MSFT and a lot of 50 jan puts on MSFT and MSFT rallies to 75 or word gets out that Bill is a transexual and the stock plummets? Where is Metooxx? Unusually quiet since this is your bread a butter -- let's hear it from the expert!
If you did it on the same underlying, you would be synthetically short [depending on strike of course, but the delta would approximate the short...] nitro