I don't think all BIPs lend themselves to this pattern. Can you think of another example?
Not right now, just seemed like a good idea to consider making it useful for more than one thing (maybe CT or something else could use it in the future?).
I also suspect each ignored flag requires carefully defined behavior, so it's probably better to spell that out in the BIP.
Sjors
Agreed, no reason they couldn't reuse the same section of the payment request URI though. (And define that behavior in the BIP)