I see it as a remote risk that any such exchange would be treated as a legally binding contract. Too many essential terms would be missing, as for example in the case of a bridge note such things as time to maturity, interest rate, size of equity raise triggering mandatory conversion, etc. While the otherwise missing essential terms might be supplied by a term sheet used by the startup as a basis of discussion, such term sheets invariably make clear that they do not become legally binding until definitive documents are signed. To say, then, that one is "in" on such a term sheet would signify no more than what one signifies by actually signing it: that is, the investor becomes morally bound to negotiate in good faith until the documentation is prepared and signed and also morally bound to participate when such documentation is prepared in good faith and along customary lines. Of course, beyond the legal hurdles, there is the not insignificant issue of how either an investor or a startup would be treated in a tight-knit community such as YC if the person becomes known as being "sue happy." Could such lawsuits ever come about? Yes, and there may be unusual cases where the parties had had a true meeting of the minds, or where one party relied to his detriment on the other's statements and conduct such that a court might find an enforceable contract, but the practical risk of this happening would be near zero for almost all such cases - enough so, I think, that it may be effectively disregarded.
What the protocol does do is help prevent misunderstandings, inadvertent outcomes, and the occasional deliberately weasly overreaching that can occur with informal verbal exchanges. It says to the investor: do I really have a commitment from you such that, should you back out, your reputation will take a hit? It says to the startup: pin your investors down to the point where you can't be double-dealt but also make sure the commitment you think you have is real and not a product of your own wishful thinking.
I can see this working beautifully as a first step within YC. Beyond that, it will work as people come to terms with its existence and see it as useful. For its purpose, it is really an elegant solution to a knotty problem and therefore worthwhile.