Yeah, I'm not really thinking about 'ard boyz, since that's a different ballgame (although multiple units of 'ard boyz is not an uninteresting prospect).
But with regular trukk boyz, if the trukk explodes, half the unit will take wounds. Allowing for the fact that you will likely save one, you are left with 7 out of twelve, or 6 out of 10. Not sure this is a big enough difference to warrabt the extra expenditure, especially if I'm taking 5+ trukks - wouldn't I rather another trukkload of 10 to potentially hit the enemy lines unscathed?
Add on to that that there is a reasonable chance that after the explosion the mob will fail one of the two morale tests, and either run away or suffer a couple more casualties, I'm thinking that a trukk that gets popped is essentially a written off mob now.
Of course, this isn't true if the trukk is wrecked/immob., but it seems that more lists are able to ap2 pen trukks than ever. Maybe that's just my opponents, I've only had a couple of test games in 7th so far.