No sensible reason 
I got a supercharger to go with my BOV, because PHUH-TISHHHH 
I know what you mean, I had an el cheapo BOV a few years ago and it gave trouble when an o ring went but one works well so far. Only a little maintenance needed.
Apparently a BOV is to release the pressure in the manifold when the throttle is closed to avoid pressure on the butterflies and stop it being pushed back out through the impeller and upsetting it.
The BLV just caps the max boost for safety, apparently.
