3. What do you mean "finalized spec"? We are evolving and adding new features all the time (
spec is on github) - how is that bad? We have a spec for Smart Property already in v1.1 of the spec, but are open to modifications / bug fixes if needed, and are coordinating the information flow between all the developers.
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.