I'm not convinced that a tiny cell of the Thing can assimilate an entire human. If it was that easy, all the Thing would have to do is spit on someone. I mean, every time it transforms the amount of blood, goo, slime ect comes out like an out of control hose. The way I see it, the Thing has a pretty small life span. The smaller it is, the shorter it lives. So a single cell Thing would be dead by the time it assimilated enough cells to control a human host. The only way it can prolong it's life and reproduce at the same time is through assimilation. If the Thing cannot assimilate something within a certain amount of time, it will die. This is why I think the Thing willingly freezes itself after it crashes.
This is what I use to explain the Thing's sudden spontaneous attacks. If all the Thing wanted to do was hide, why would it attack in an environment where it's clearly at a disadvantage? I see the Thing attacking in these situations because it needs to assimilate in order to survive. Other times it feels the need to sacrifice a big chunk of itself so a part of it can get away and assimilate someone else like the tied up prisoners.
Again, I know it doesn't match up with everything, it's just the theory I use.