I've been snooping around for a while now, and while there's no lack of reasons to stretch them, no one seems to be training their hip flexors. What's the reason for this? Because they're a smaller internal muscle with no cosmetic impact? Because they'll never have to do anything more than raise a pair of legs? Even so, it's still weird to me to ignore them entirely.
I am active in aerial arts and gymnastics communities. In these disciplines, we train hipflexors all the time (although most exercises that use hipflexors hit abs and/or quads as well), since powerful hipflexors are necessary for a fast and tight pike fold, as well as some other similar body shapes which are frequently used in these disciplines.
Most of the hipflexor exercises we do are variations on hanging or supine leg lifts.
It's not actually that uncommon to train them. Most ab exercises work the hip flexors as much as they do the abs. There probably isn't a huge benefit to having them be extremely strong, but powerful hip flexors (i.e. able to exert force at high speed) would be beneficial for sports that involve kicking,.
I have a set of spinal injuries and one of the things I cannot do is flex my left hip well – it is innervated by the L1/L2 Myotome.
Trust me, if you can't flex your hips, walking becomes very difficult in very odd ways – and almost every exercise you can do that remotely relates to either walking, holding your legs out straight, raising your bottom from a flat surface while lying on a bed, or doing a sit-up uses the hip flexors as part of a complex movement. I think fitness people don't train them specifically because there is a lot of cross-over from other activities. A lot of leg-specific exercises (like the clams) work them extensively (and I can only really do those exercises on one side as a result).
Having done a bit of powerlifting, I've found training them, especially in isolation to be useful in strengthening squats.
It helped me be better about keeping knees over toes and driving straight down rather than knees caving in slightly. Most of the stronger people I've talked to (other powerlifters, strongmen, etc) tend to concur.