I've never packed and I've never felt the need to. Maybe it's because of my weird torso/hip shape, but nearly all pants I've ever owned (except for the men's cargo pants I'm currently wearing) fold up on themselves towards the top, making it look like I have a bit of a bulge. Even skinny jeans from the girl's department did that for me. But at the same time, my boyfriend never really has a noticeable bulge, even when he has a boner, and he's average sized in that department, so having a bulge certainly isn't required to look male.
My bottom dysphoria isn't that bad, and I think packing would just (quite literally, depending on the pants) rub it in that I don't have anything down there, making the dysphoria worse. That's just a guess based on how I've noticed I respond to things. I'm fine normally because I don't notice my crotch, but feeling something added down there would make me notice. Once I got used to the feeling of wearing a packer, it would probably feel weird without it, though. Of course, I can't really try out a Mr. Limpy to find out for certain, because I don't have a way to order stuff and have it shipped to me.