I think that often the artist is not being lazy but frugal. In my case, I've created skins that have every, or very nearly every, bitmap made fresh.

I've also created skins that have many repeated bitmaps. If the skin design really calls for it I design all new bitmaps for it. Sometimes, though, I will use the exact same bitmap for one thing. For example, my skin
Spander uses one bitmap for all the titlebar buttons. My original upload to WC of this skin had one bitmap for each button, but I realized that it was a waste of Kb's. Remember, not all people have DSL connections. Most still have 56K dial-up. By changing my
Spander skin the way I did, I was able to shave 60 Kb's off the size. To someone with dial-up, this is a plus.
Point is, I only create new bitmaps if they are essential to the design. Buttons, titlebars and borders with bitmaps showing their dormant/inactive states are superfluous in my opinion. Who cares what they look like when they're not being used? I don't!
If there are ways to make the skins file size smaller then why not?