Do you understand a concept of hard fork? Each update to a set of rules needs to be done in such a way that all clients will be updated before new rules are in effect.
If two client implementations can disagree about a results of a transactions, it might be used for double-spending.
If you do not have an update schedule, clients will be updated at random points and they WILL disagree.
This is what I meant by "finalized spec": you have to do a feature freeze at some point if you want Mastercoin do be secure.
Alex, you have known me for enough time, I would expect you to rightfully assume I understand hardforks. In any case the answer is yes.
I don't think we need a feature freeze, we just need to think more carefully about the spec changes schedule.
Thanks for bringing this issue to our attention, I'll send it to our developers and J.R. for consideration.