I agree with you Kid. If I was in an MP game where another human was sending thousands of troops into my land, I would cancel the deal quick smart. In fact....I think someone is doing this in my current game!!!!!
On a related note, I think there is a good argument for a three-tiered border system-Closed: which only allows in invisible units; Open Borders: which lets in workers, settlers, scouts and missionaries and Right of Passage: which lets in all units. The benefit of OB in this instance is the power to keep out military units, but at a cost to your economy (trade routes worth less) and diplomatic relations (you get a bonus, but not as big). A full RoP would get better trade routes and diplomacy, but at the cost of letting their military units pass through your lands (and possibly making you a target for war from another neighbour!)
Aussie_Lurker.