Thanks are worth it… if you can manage to pile up a ton of them. They are worth 2 infantry so if you manage to use them in more than 2 attacks you will have worth for your buck.
And most of all, if you manage to pile up lets say 15 tanks most of the defensive strikes will bonce off your tanks and if you manage to attack without taking any hits… your enemy morale will wither away.
I normally start to invest massively in them if I end up loosing air superiority for good. And I manage once to turn the tide whit this strategy (using central powers) but I must admit that it does not works every times (damned planes)