Might seem a little picky but you can't put an empty drafting view on a sheet. You get this complaint/warning if you try.
If you had the idea that you could block out a few sheets with placeholder drafting views in your template, sorry...not if they are empty. A little text, or a drafting line, a symbol and it won't be empty though so you can fix it and get nearly what you want!
What if you remove everything from the drafting view after you put it on the sheet? Revit leaves the view on the sheet without a complaint. Tricked!! Add something to the view, put it on a sheet, remove the something from the view...empty drafting view on your sheet. You can even use activate view to remove the something from the view and it won't mind. It's just the initial putting it on a sheet that Revit complains about.
Technically it's true of other view types too. Before you put it on a sheet, try turning everything off in a plan view and then putting it on a sheet, sorry, same warning!
[Added 8/15/2011] Andrea with The Matrix Group, an engineering firm, wrote to let me know how they deal with this. She wrote:
The tactic we use for “blank” views is to place a single reference plane in the drafting view. We never print reference planes, so they won’t show, but will allow a placeholder on sheets.