I suspect that sometimes its just easier to set the main road to PLot than it is to find all the ramps. I believe that the initial guidance to set the entrances to PLot was agreed to before the current transition penalty for PLot was put in place. When there was a penalty based on the number of segments, setting the entire road to PLot wouldn't work, as you would always be routed off of the HOV lanes at the first available exit. Under the current scheme, I don't really see a big difference between the two.
Another issue to be aware of is reversible HOV lanes. With reversible lanes you need to set both the entrances and exits to parking lot roads. This is because if any entrances are reversible along with the main lanes, they become exits when the traffic is flowing the other direction. Once you set one exit in a direction to PLot, you need to set all the exits in that direction to PLot.