In my experience, the explanation has always been tied to the ministry - we want to present as clean an image as possible. I always pointed out when reasoning on this issue (in favor of the WT stand, of course) that American politicians almost never have beards. Why? Because they want to make sure they get the votes of everyone, even the people with a reaction against beards.
However, in the minds of Witnesses, whatever reasonableness is here seems to become hardened into a superstitious fear. My friend told me that when he began to grow a beard lately, a person at the hall took him by the arm and implored, "Brother, stay in the Truth." Similarly, my old roommate came by my house last week to finish some work he started a while back, and although he didn't make conversation with me, he was visibly shocked by my goatee.
JWs: it's facial hair. Grow up.
SNG