That makes for an ineffective threat and takes away the ability for offense to spread out the defense, decoy and greatly limits the offense in general as it makes it more predictable. Plus it eats up a roster spot where most teams carry 5-6 WR's. Now we'd need another roster spot because we have a WR who can only somewhat WR.
Doing so is more of a hindrance. Especially when we've seen UDFA's like Lance Moore do similar or better with a full route tree.