Not at all. Terran gets mules because protoss gets to chronoboost probes and zerg gets inject so they can both outproduce Terran in workers. So if mules mine the same amount regardless of where you drop them, it's actually disadvantageous for Terran to drop them on gold precisely because you are mining it out faster without the faster income the gold -should- be providing.
While you make a good point of mining out gold bases faster, it isn't really that much of an issue.
If you accommodate to the influx of money you will get then it is still a bonus. Sure it is only temporary advantage, but I still think your dumb not to drop mules onto a gold patches for the pure fact you don't mind to mine it out fast.
Think of it this way, TvT both players are just mining minerals and producing marines / barracks, and that is it, both macroing perfectly, the only variable is that one Terran player is dropping mules on the Gold, where the other isn't.
During the Mules being dropped, the gold muling player has a temporary advantage as his income will be greater than the non-muling terran, thus giving him higher rate of production, which then leads onto more food towards units and, should we say, a slight timing attack to hit when he has a slightly greater food count.
Of course the advantage will run out and will become even again, but for a short amount of time, there is a slight advantage in the Terran that is popping mules on the Gold that I really think it isn't too much of an issue to worry about mining out a gold base quicker than normal.
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