Good question. A producer can survive without a rapper but its difficult for a rapper to do shit without a source for beats. But I believe it's harder for a producer to get the name out if they ain't hooking up with rappers (cause at the end of the day, isn't that one of the major points of hiphop production?) So, rappers need producers but emcees are useful to producers...so both really need the other.
However, there are some producers that don't need rappers because their beats speak for themselves (Shadow, RJD2, etc).
It's really a matter of opinion and what the producer wants to be doing....