Essentially the thread title states my feelings on the matter. The land titan doesn't really fill a useful role. Its high durability is offset by the fact that the bot titan does a better job of killing units and has basically the same mobility. There's a lot of overlap in the roles, and there really shouldn't be. The role we've needed since beta was a mobile planetside anti-orbital unit. We have a ship that can do it now, but it's limited by water when many maps don't have enough to justify a naval factory. The tank titan could have been so much more, and I honestly feel it needs something to make it more relevant. Making it a supreme anti-orbital mobile fortress (Bolo anyone?) would change how late game develops. If it could also build anti-nukes to cover army movements and prevent nuclear army wiping, it would make a lot of people really happy. The nuking of armies every time they move away from anti-nuclear defenses and the lack of any way to resolve it has been a frequent complaint for a long time. Anyhow, I just see too much role overlap and not enough reason to use the unit. And since there's a perfectly needed role that it can fill I don't see why it was made the way it was. If there's another way to deal with army nuking that I'm not aware of, I'd like to know. So far splitting my army up and having both dodge when I hear the nuke alert hasn't been that successful. Especially when I get closer to their base and the travel time for the nuke is really short.
mobile planetside anti-orbital unit is not only the ship... http://pa-db.com/unit/bot_tactical_missile
It outranges just about every other ground to ground unit in the game and it's direct fire with a fair bit of AoE. It's better than either Fatboy due to the massive amount of HP that it has. Don't use it like the Atlas, use it like an army of Gil-Es or Shellers and you'll get better usage out of it.
Titans are only good if your opponent doesn't know you have one until it's too late. Otherwise if you die to one there's no excuse for not being prepared.
The Zeus dies easily to a handful of fighters and has hardly any range. It's only a danger if you're totally unprepared.
