Thanks for the replies so far guys.
Yes Silkster, I feel that blocking the exit by piling it with units/wrecks is wrong. If the exit hex is in terrain not passable to some units, then 12 or more wrecks effectively makes the exit hex only usable to infantry-type units. Just because it's possible to do something does not make it ok. Also, unarmed (truck, etc) units would have no chance of entering an exit hex that contains even a mere halftrack enemy unit, though I have a (slightly) less of a problem with that. Also, I fully believe that the area NEAR the exit hex could be defended vigorously (with covering arcs of fire and lots of units ready to op-fire).
The origin of this rule is actually from an article I read in an old issue of "The General" where players were pulling this, and since it had also happened to me in panzerblitz and panzerleader, we (my friends and I) implemented a rule against this tactic, which I in turn carried over to this game, between myself and my opponents only.
Some may feel that "alls fair in love and war," but I feel that it is not fair, and not realistic either (there's that word again!), as in real life, the enemy would simply drive around the hex. Anyway, I (originally) was explaining that this "tactic" was the opposite of "camping" reinforcement of entry hexes, and a discussion ensued. Y'know, the ol' "shouldn't halftracks be able to such-and-such" kind of topic.
I'm mainly just trying to get an idea of how people on the boards think. As far as how I handle this topic? I simply cover it like I do any other - I check with my opponent before starting the game. I doubt anyone I game with would try this on me though, so it's kind of a moot point in a way, but still fun to talk about.