I think a part of it depends on what breed or discipline you are involved in. Most of the horses that I see on here are left with the forelock, though I have seen a couple with it roached as well. When we trim our mules, we cut the forelock off.
The Saddlebreds who have roached manes (3-gaited and equitation) always have their forelocks roached. Andalusian mares almost always have roached forelocks, too. Polo ponies always have them roached as well.
I roached my gelding's mane and took his forelock off. I personally think it looks better without a forelock... especially if the horse has a thick forelock.
I think the only way I would be able to roach them completely(forelock off) would be if they have a sucky forelock. I like Dozer's all long and a lot of people tell me not to trim either of my horses' forelock even though Bart's is so wimpy.
I don't do either to my horses, since they have trouble growing long manes/tail in the first place (except for Pecho). But if I had a horse that was bred for it like fresians and what not, I have to say, I probably wouldn't take off all the forlock, just enough to clean it up and make it look nice and sharp instead of poofy. That's just me though, I'm not sure how they are required for shows, they probably would have to come completely off I'd assume.
I have never actually roached any of my horses so I don't know if I would leave the forelock or not. I think it would depend on the individual horse. If he had a nice forelock, I would probably leave it and if it was short or scraggly, I would probably shave it too.