A NMC block is much easier to find than a BTC block. Each time a NMC block is found, your work is being interrupted by a LP message. The current work and queue needs to be dumped and new work needs to be fetched. If your miner is slow, you might be interrupted relatively often.
Relatively often is all relative. NMC block is roughly 2.4x as easy. That is an extra 1 to 2 LP per block. When NMC merge mining was new it was more of an issue with 20 to 30 LP per block.
The LP gnar1ta$ was refering to (5 per share) had nothing to do w/ merge mining. p2pool uses larger shares. 1 p2pool share ~= 150 difficulty 1 shares. It also builds a share chain to continually recalculate reward split and roughly every 10 seconds a new share will be found by someone on the p2pool network and thus requires an LP.