TH Tigers are, I'm sure you'll all agree, very expensive to build, compared with everything else in the game. Regardless, people still have more than enough money to amass armies out of nothing but TH Tigers. They can mine everything they need from their home mine, just build 2 or 3 smelters and its all good, they need never leave home. This not only detracts from the gameplay, but also isn't how the developers wanted the tigers to be used, I'm sure.
I propose that the mines will eventually drop down to a production level of about 10 or 25 ore per load if they are mined out for too long, thus no longer making them "bottomless". The time it takes to mine them out depends how many bars it is. This makes tigers much more costly to build, the way they were supposed to be. Panthers *GASP* might be useful, since the tigers would only show up here and there, and you'd need something with more beef than a lynx. Building 93 smelters in the same area begins to lose its charm if the mine runs out of ore just as construction of the 94th one begins - particularly when you had to spend all those common metals to build the smelters in the first place. It would greatly add to the "s*** we can't afford to screw up" atmosphere that would be in place if you were the last 200 humans in existance, and you were living on a planet that pretty much wanted to mash every one of your bones into paste, like New Terra basically does. Spamability would no longer become an issue when proposing unit ideas. And about 40,000 other problems would be instantly vaporized.