You say that, but...
I'm currently toying with ROMs on an old Motorola Bionic, which serves as the central music player for the stereo in my garage.
I've waited minutes for ART to do its thing on a singular package, but I've never waited minutes for Dalvik to launch the same thing thing on a properly-working device using Dalvik.
They are therefore not the same process. You might think that they should be, and I might agree that I think that they should be, but they're simply not -- or at least, the ART compilation process always takes longer than the Dalvik JIT process.
(Also, you sound a lot like the folks responding to those who question memory management on Android: "It's taken care of automatically," they say. "You can't do anything to improve it," they further proclaim. But there's a dozen or so tweakable parameters, and nobody can tell me that they're perfect for my usage out-of-the-box than anyone can tell me that my "swappiness" parameter on a Linux box is sane, out-of-the-box, for what I'm doing with it. It may be a reasonable default, or it may not be. But toeing the line unquestionably on the published Truth Of The Matter, and Accepting that Truth for What Is is a foolhardy thing to be doing. It may also be the fact that this same Bionic had a long string of bad Dalvik VM parameters that lasted for months or years, resulting in general sluggishness and frustration, though the 12.x Lollipop-based line seems better for reasons I haven't yet bothered to investigate.)