In XL mod you can build sherwood foresters as the english that have similar effects. I personally dont like the unit types as there 12 man size dosent equate to much punch.

As scurvy said, you can use thier stealth for advantage but to what end? I suppose, as sun tza would say, in warfare it is best to make your strategy. For this reason hashashin work because the AI will adjust to them once they are spotted.

Placed at the right place they can set up a decent ambush. Try setting them up near a tree line with some hidden units within and maximize the effect. In all honesty they dont make it onto my unit roster often.