Why should I not do this?
I can think of a few reasons. But they're ... nuanced. I'm sure I've covered this before, somewhere ...
The comfort thing is a Catch-22. SD-EZ Grip is uncomfortable at first only because you're not used to it. And you won't get used to it unless you get used to it. It's a vicious cycle, that you need to break by deciding that your temporary discomfort is part of the normal learning curve. And before long you won't even remember the problem.
It's like guitar. When you first begin to play steel string guitar, it hurts like heck. Then it stops hurting, because you develop callouses. But if you give up because of discomfort, you never develop callouses. If you give up on SD-EZ grip, your hand will never learn to stretch properly.
One of the hardest things to automatize on whistle is the role of the bottom pinky for balance and support. It may take years for your pinky to learn to dance properly. And pinky grip short-circuits that process by forcing your pinky into a different role. If you never plan to play high whistles, maybe that's OK. Otherwise you're only hurting yourself in the long run, IMO.
It's true that on large whistles, the ring finger on the B3 hole may take over that support role. But then you're mentally coupling the ring and pinky fingers to act as a support unit. That's quite different from coupling them to act as a fingering unit. For effective pinky grip, you shouldn't literally rest your ring finger between B2 and B3, but move it in parallel with the pinky, otherwise you're hobbling your fingering dexterity. And this trains your brain to a very different, and more limited, neuro-muscular pathway.
All uilleann pipers and many fluters use the SD-EZ grip on their low-whistle-sized instruments. And you almost never see this discussion on pipes and flute forums - though flute grips can be a matter of debate. But people know how it's supposed to be done and they just do it. So why this issue keeps coming up among beginning whistlers (and it does) frankly baffles me.
Sure, as the Perl motto says, TMTOWTDI - there's more than one way to do it. There are always players of any instrument who do very well with idiosyncratic techniques. When I first started low whistle, I discovered pinky fingering and thought I'd found the Holy Grail. Until I realized the disadvantages, not to mention that there's no need. Personally, I can switch easily between either, but I'm a very experienced wind player. As a sometime teacher and advisor, not pressing beginners to learn proper EZ grip (OK, piper's grip) simply seems to me a dereliction of pedagogical duty, or something like that.