Tanks are useful, you it's more military stuff in general, and even though they fall to planes, it's good to have them. Let's say you're attacked, if you don't even have any tanks, your opponent has an advantage, because they can bring in tanks at a better combat rate and defeat you, but if you have tanks, then they have another factor to worry about. Additionally, if you're gonna worry about score, remember, you can only have 5 barracks per city, so 15,000 troops per city, that's a limit to 7.5 score, per city from soldiers (correct me if I'm wrong) whereas you can still build 5 factories past that, with 250 tanks per factory, that's the ability to get an additional 750 score max (again, correct me if I'm wrong, I don't trust my mental math here), so, yeah, they can be worth it.