Is the theory that taking more shots lowers your fg%? Because the argument can go either way. Defenders might concentrate on you more outside the line, but there's also consistency from taking the shot more often.
If I understand D.o.s. correctly, the idea is that taking a lot of three pointers, generally speaking, indicates a greater capacity to take them. Otherwise the player wouldn't get playing time.
Capacity in this case can mean a lot of things, including the consistency you mention. It can also be a J.R. Smith kind of capacity, where the player is inconsistent, but when his shot is falling, it's REALLY falling, and he can nail a bunch in a row regardless of defensive pressure. There are a lot of reasons why a coach might be willing to let a guy bomb from deep, but most of the time it'll have something to do with his ability to actually put the ball in the basket.
My point by bringing up Millsap versus Lowry is just that I think if you're going to judge how good a three point shooter somebody is, you have to consider the context, which includes a number of different factors. Makes versus misses is an important as a raw measure of success in shooting, but it's not everything.
With Lowry specifically, I think his percentage would be higher if he weren't taking so many shots. On the other hand, I think part of the reason he takes so many shots despite not always hitting a high percentage is that he often plays in lineups without many reliable three point shooters, so his team needs him to bomb from deep. That may change this year if they play Carroll at PF a lot of the time.
The lack of other three point shooters on the floor is probably part of the reason Smart took so many deep shots last year, too. The Celts had a good number of shooters, but Smart shared the floor much of the time with Turner, Bass, Zeller, Sullinger, and Crowder -- guys who were non-shooters from deep, or close to it.