Bryan,
If you're doing large venues, and your speakers are 10 to 20 feet apart, you need a monitor. The wedge on the floor is just what the doctor ordered IMHO, and without it, those big jobs are difficult at best. For smaller jobs, when the mains are positioned relatively close and somewhat behind you, say 3 to 4 feet on either side of the keyboard and about three feet to the rear, you don't need a monitor. If your EQ's are properly set, you won't get feedback, especially if you're using a high-quality, super-cardiod mic. You will experience feedback if you use a uni-directional mic and it's pointed at the speaker. The super-cardioids usually have a very short range, which also helps eliminate outside sources of noice from entering the mic.

Hope this helps,

Gary