Actually, Jirachi comes in handy in that it turns wasted turns (Don't have enough energy to attack, don't have your Stage 2s yet, etc.) into somewhat helpful turns. It takes no energy to attack or retreat, it gives you a card when it is KOed (though not if by Machamp's Take out or Gengar's Shadow room :[) all of which mean you're essentially losing nothing by using it except for your attack of the turn. And if you didn't even have an attack to start with....
Once you can attack, it switches out the easiest of any set up card (except Chatot) being that its the only one with free retreat and no energy cost (and it doesn't nuke your hand like Chatot, which sometimes is useful and sometimes isn't)
Works very nicely in Flygon builds, except for the fact that I reeeally don't need to be handing Gengars free prizes <___<