For me, tower defense is supposed to be this hard. It's about losing over and over and refining and revising your strategy each time and getting further, like a kind of puzzle, I guess.
There isn't much more pointless than easy tower defense, because at that point it's basically a game playing itself!
I am getting my butt handed to me in geoDefense Swarm hard levels, playing them little by little to not lose my sanity and trying to avoid watching YouTube solution videos.
My problem with Defense Grid is I have a hard time resisting speeding up the level and then when I lose too quickly I get annoyed.
I think tower defense has a lot of template code out there on the internet (like...full working source code up for grabs) which is one of the reasons a lot of them come out, if people did something a little bit more interesting I thought they would sell better but the game that combined zombie action and hillbillies didnt seem to do very well.