WAR isn't useless, but it isn't an end all be all stat either. It should never be used alone as a way to judge a player on his "value" to a team.
Of course it is far from perfect, though I think it can be useful when you are looking at alongside numerous other stats to sort of paint the piture on a players overall value: Including OPS, OPS+, BABIP, Range Factor, UZR, wRC+, wOBA, K rates, BB rates, GB/FB/LD rates etc
Assuming he is healthy, Mauer will be fine, I don't think he will be some 1.000 OPS guy consistently ever, but if he sticks in the .850-.890 OPS range while playing C he provides plenty of value.