This reminds me of a screenie I saw in another thread where the player's starting settler was boxed in by mountains and couldn't move. IMHO it's not about how often it happens, it is the mere fact that it's possible that worries me. Sure, if it happens to you, you can just regenerate the map, or load up the 4000BC save and settle on the coast (in the first example). But if it happens to the AI, you won't know about it until later, possibly much later, and you effectively have one less rival than you should. And, of course, the AI doesn't know any better than to settle in place.
These problems could easily be solved btw:
- Allow passage through mountains (maybe only allow this after a worker has built a road through the mountains).
- Allow cities 1 or 2 tiles from the coast to build ships. Their BFC touches the coast, so why not?