What you say is true. And its effectiveness would be unquestioned if you didn't have the 3-point shot. But with the 3, and with how Delon is so good at finishing at the basket (he shot 63 percent from 2, which led the Pac-12), it makes little sense for him to develop a mid-range game unless you knew that was the only shot he could get AND he could never crack 22 percent from 3 (which is what he shot last year). And at 6-5, that 10-12 foot shot should be there every time anyway, but as we all know, not all defenders are created equally, and part of the reason he shot 63 percent from 2 is because guys really cannot defend him straight up. So why help the defense by asking Delon to take lower percentage shots?
Also, it bears repeating: Delon is a master at getting to the line. He had nearly 200 FTAs last year, which from the best information available, might be a single-season record at Utah for a guard (The top 10 in the media guide only goes to Doleac and Nevill, who each had a 215 FTA season; Delon had 193). Shooting more mid-range jumpers takes away from that, and again, bails teams out defensively (fewer fouls, etc.).
The 3-point shot, however, is an obvious difference maker. It's worth 33 percent more than any 2-point shot. We're all familiar with that. Delon and Utah would be best served if he could get his percentage above 30 percent. Now, a lot of those 3s will come late in the clock but it doesn't matter -- if he hits 1-2 of them, it does far more damage to what the defense has to do than if Delon drains 5-6 mid-range jumpers.