SEQUENCE is used as a synchronization aid and as a change indicator.

However as a synchronization aid we already have DTSTAMP.

However as a change indicator there is already a requirement that attendee CUAs 
have to spot significant changes that are relevant to their calendar user, so 
SEQUENCE is not important.

Proposal: deprecate use of SEQUENCE.

At the very least if we choose not to do that, we should be much more explicit 
about when SEQUENCE is supposed to change (e.g. ORGANIZER MUST change SEQUENCE 
for these things (...) and SHOULD change SEQUENCE for these things (...) under 
certain circumstances.