perry, you must have mis-read the label. The old Dexron III and Mercon fluids were in fact the same thing. Now, since GM no longer licenses Dexron III and Ford no longer licenses Mercon, the aftermarket is calling them D/M or DIII/M or DexMerc. There's no way the label said not to use it in a transmission that specifies Mercon, although as stated above you shouldn't use it in a transmission that specs Mercon V.
Anyway, I second what roadrunner1 said ^^^. Ford now states that Mercon V is the proper fluid to use in all transmissions and P/S units that originally spec'd plain Mercon. You can still use the DexMerc stuff, but you'll have to change it more often.
On a side note, I also questioned why they originally said it was a big no-no, now they say go ahead and use it. I bought a couple of cases of plain Mercon before it disappeared and I've been using it where spec'd but sooner or later I'm gonna have to make the switch...