Actually if you study the forces at work the seat tube is one of the harder working parts of a frame. The viability of riding this frame will depend a lot on how big of a dent and crack it has. You haven't said anything about the size of the damage in terms of how big the dent itself is, how deep the dent is or how long the crack is other than "it's pretty long already". Depending on how you see such things this could be anywhere from a half inch to 3 inches. So the opinions on the damage, other than the gut reactions given so far don't really mean much without knowing some measurements of the damage involved or, better yet, seeing a picture.

Also being a mountain bike means that the damage could have occured from a nasty crash onto a rock. Or it may have occured from a car impact. Either way you would want to check the frame out carefully for signs of deformation in the alignment of the tubes. And that isn't the sort of thing you can do by eye. It requires at the very least the use of a couple of long and very good straight edges along with careful measuring.