Hello!
You can get PA space from a LIR and announce yourself, but this will usually lead to bad routing and in case of the PA-providing LIR going down for whatever routings, partial connectivity loss. So it's not providing the same "quality" like PI. If you're interested in the matter, google for "IPv6 multihoming" and find a large amount of long threads discussing this problem space. :-)
It may be if you are trying to announce the subnet when all net announced other way. But if LIR gives you an own /32 (like /19 for IPv4) that only you announce it into the Internet with your AS - how it can interact with other net an AS that are does not interact other ways that they are got from one LIR? In addition, I asked a question on LIR courses about what will be with assignments if LIR goes down, for example because of no payments. The answer was "all assignments will be exist". Also LIR can't change or delete objects does not protected by LIR's mntner, so LIR can't delete or change their assignments given to other companies. So I see no reasons to become a LIR if you need only IP/ASNs (even v6), but not registration services, as well as you don't need to become the registrar for registering your corporate domain name (or even few corporate domains). ;) -- WBR, Max Tulyev (MT6561-RIPE, 2:463/253@FIDO)