I think many people can agree here that they've had this problem. This way you know how many lines are in that template and can easily find that line in the editor. I have not seen any software forum,CMS,blog etc... thats implementing it and it would really save time when your making templates or changing templates.
I know I sometimes explain things badly and pictures are worth 1,000 words. Of course it wont have all the other stuff just the column that shows the number of lines. For implementation I think doing it client site would be easier then adding the load to the server.