Except in the woods, skirmishers may be usefull when both armies are close and within striking (melee) distances – wisely used they can prevent enemy from closing in and initiating melees.
Also when you are marching by road during the night and know that enemy may be very near, cavalry unit on the front of your column is able to move one hex then dismounting and let out skirmishers, and so on all the night. March is slow but it's better than doing nothing (I used it several times on the wooded battlefield of Corinth).
The biggest problem is that that ‘skirmisher cost’ one time is working and other not. I don't know if it is deciding randomly but sometimes unit with 2 MVP left is able to move into hex under skirmishers control when other unit is paying for it 5 MVP.
I was testing the usefulness of skirmishers for some time and this topic prompted me to send something to HPS Support for an explanation...