By tap off, I mean people walk up to the tapstile, touch their MBs and don't go on the ride. But people would not do that for a Tier 1 since those are the rides most people want. People make Tier 2s they don't care about. You can either let them expire, like you said, or tap them off so that the two Tier 2s can be used quickly so they can get another Tier 1 earlier in the morning when there are more available. People who don't go at RD sometimes book then when they know they won't be there and let them expire.
We typical have enough Tier 2s that we want to go on anyway so we don't do that, but it's a strategy people use.