I've seen several apps that aren't seriously meant to be used - are there any ones which are meant to be used, and have actually been used by players?
At our local LARP group they use a dice roller App (I think it was that one) and they have their characters on a wiki (I think it is a tiddlywiki) that formats nicely for mobile. They keep all their stats there, track of damage taken between sessions and perform random rolls for magic.
They do not use them for LARP combat (which got pretty hairy). I think most of players favour little faux-leather notebooks to keep track of stuff. Not only does a rustic notebook fit into the game world more neatly but there is a worry that the phone would be lost or damaged (I certainly was!).
The ref used his phone for keeping notes - it saved him writing them up at the end of the day.
Each time I've played LARP, the rules have been quite different. Combat is mostly the same but for magic use, healing and so on I think it might be quite tough to write an app that covered them all. Most LARP groups have plenty of house rules that suit the way they play. Tracking XP can probably be done but I think a one-app-does-all might not be all that useful.