Lord Thane I think from a 1 on 1 perspective, thats a completely acceptable premise. But we know it isn't a 1 v 1 kinda game. That information is going to be relayed to someone who would be more than happy to drop a truesight on you. Someone well above your own ks wouldn't have to commit the same fraction of their army to get through you.
Now when your army returns, youve got a few options. Sit in protection and build (not entirely unacceptable), hit your original targer and probably get immediately retaled, or retal the larger guy, with or without a ts.
Thats a lot of risk for commiting to an unknown attack. You could even this out a little bit, possibly by creating an icon indicating a kingdom is under the effect of SH. In that case, you making an attack knowing the risk and probability, is far more palatable.