Pay Per Last N Shares might encurage miners to continueously mine, but it will also drive away part time miners.

One of the reasons Eligius is popular is because it offers free PPS rewards.  This is atractive for its low variance, but also to part time miners, and as a backup pool, and for pool hoppers to come to on long blocks as a fallback pool.  Switching to PPLNS could drive away all of those users, and their hashing power.