Sherwood Foresters are quite expensive to train, 600 florins for just 20 men, so I don't think you should train them unless you're financially stable. That said, their combat skills are excellent, and are great for flanking the enemy, such as generals as Martok said. I use them in my Irish XL campaign for melee, and see their bow and arrows as just an added bonus, using the ranged attack only when I know the battle will be short since I don't want to tire them out and only kill several men when they can kill many more in hand to hand.

I would always have one or two in my armies, trained up with the best armor in Mercia since they're expensive to recruit. I'd also stretch them out to a single line to maximize their engagement line, which results in more valor for these killing machines, able to slice through any unit despite not having an armor piercing attack. However, since you have Highlanders, I don't think these units are crucial, as the Highlanders have more men and better attack against heavy armor than Sherwood Foresters. Nonetheless, I'd put some in my armies, since I prefer quality over quantity, even on Huge unit sizes.