Doesn't stop a third party share - namely, if I give my account to you, what's stopping you from giving it to other two people (and consequently running down the number without my knowledge or anyone else's for that matter?) and what can I do to stop it?
There'd be no way for me to track this, and that lies in (most of) the problem. At least with the X360, there was no real way you could 'outrun' the acount the way the PS3 can.
The other half is of course, proving people will actually stump up. Generally it's just due to the fact that well, to get the DLC only one person has to pay. So you could (and I've seen this happen) have cases where people promise to pay in, then once they get access to the account, lock it out for use with everyone else. This can be devistating if it was run at say... catalog 15 or so.
So combining the two, you're asking for some serious trust, and I'm not entirely sure if people are willing to actually put their money where their mouth is.
The best way to minimise damage from a security perspective is fairly simple though - you immediately launch with 5 people (in short, it becomes nearly impossible to steal the account) and you get a member (or even better, a third party) to actually have control of the account, and to issue monthly password changes and to manage all funding. (And from memory all email changes MUST come from the originating PS3 due to PSN security changes.)
Essentially what happens if anyone DOES try to do a runner, they can't take the account, and they get locked out of any future releases - meaning they can only access what they paid for to begin with.
You'll still have to trust the gatekeeper now, but well, can't fix everything. This scenario requires you to trust SOMEONE so take it as you will.
It does mean you can't redeploy any of the five uses if anyone goes rogue, but well, that's one way around it.