2 Close,
Many airlines do have a requirement of having the MEIR valid at the time of application or they will simply not consider you. For this reason alone quite a few pilots choose to keep it valid in order to keep the options open in an already small set of choices.
Your point about the SEIR and MEIR is a very valid point which one must take into consideration. The MEIR gives you privileges to fly an SEP IFR however you can't do it the other way around.