Very late on this one :P But I think the difference is the fact that ports are unique, so if the kernel assigns you a bad source port by accident then it can't

Very late on this one :P But I think the difference is the fact that ports are unique, so if the kernel assigns you a bad source port by accident then it can't change it later. You would need to know this is what happened and bind to a new source port (and do you attempt automatic again?). Where-as with overcommit, your program doesn't know or care what particular memory page you're using, and the kernel can easily fix things up in the background (Ex. use swap, or kill some other process) without you ever knowing about it.