The DEF itself is pretty inexpensive (though not free, like $8/gal), but the system to keep it from freezing in wintertime is actually kind of a pain, since it freezes at -10C but decomposes around 60C so your heating element can't be too hot. The cost savings from not using it were rather substantial. Volkswagen TDIs were the cheapest performance diesel cars by a rather large margin.

It's an active expense that doesn't directly benefit the user, and disables your otherwise mechanically sound vehicle when it runs out; both of which tend to rub people the wrong way. Also I think there was something about german companies colluding over adblue tank size to force more refills, but don't quote me on that.

Also it's a user-maintenance thing. One more thing to refill on top of your windshield wiper fluid.

I forget if it was VW or Mercedes, but one of them branded theirs as "blue-tec" or something that doesn't sound like you're handling piss bottles. Let the mechanic worry about that.