Well, you only need 17 levels to get the maximum offensive so you miss out on 1 point of AC; worth it if you ask me. 13 levels will get you +4d6. I think additional Skirmish-classes also fall thusly that you'll have an extra level or two to work with. As such, you don't actually lose Skirmish in the long run, tho you may lose some for certain phases (singular levels where it'd normally increase, to be precise).