Mustering the Rohirrim can be a very powerful card in the right deck. It's pretty much the best way to search for rohan allies (especially Eomund/Gamling), besides just straight drawing them, and should trigger every time (if it doesn't you probably don't have enough targets). It is a bummer that it adds only one to hand instead of all targets found, but is still a good search effect.
Mustering the Rohirrim
Event. Cost: 1.
Action: Search the top 10 cards of your deck for any 1 Rohan card and add it to your hand. Then, shuffle the other cards back into your deck.
"More speed we cannot make, if the strength of Rohan is to be gathered." Éomer, The Return of the King
The Hunt for Gollum #7. Spirit.