With proportional you have a 'clean slate' after each round.
With shared MaxPPS you potentially accumulate a big 'debt' across many rounds. Which, of course, would average out eventually, but people may not want to stick around that long.
That's a misconception you have. With either proportional (current method) or SMPPS,
the pool will not pay out more than it has actually earned. The difference is davka on
shorter rounds, where the proportional method pays out immediately, whereas the SMPPS holds some of the funds until longer rounds. This is precisely how it deals with pool-hopping, BTW.
So, if there is any 'debt' in comparison with the current method, it's actually only accumulated on
shorter rounds, and payed back on longer ones.
Hope this clarifies.