If my .02 is worth anything, I think the default banners are valuable enough to keep. They (of course) act as place holders and have the formatting/sizing necessary to model future banner additions.
The issue I had was the expiration function did not appear to work. The banners continued display after the expiration date.
If they could be disabled, it would allow for future use without them interfering with their regular display on the page...a check box disabling the banner is an alternative to the expiration date (but is not apparently available).

I've just resolved that issue, although it's a minor bug. If there are no banners on active campaigns, it reverts to showing default banners and didn't check the expiry of those.

I agree with Doc Natural "I think the default banners are valuable enough to keep. They (of course) act as place holders and have the formatting/sizing necessary to model future banner additions."
I find that place holder important during the themeing process as a solid reference point. For me, Chris' repair of the minor bug is sufficient at this point.

Feedback taken on board, just killed the "hosting" banner, but kept the "donate" and "advertise here" banners. "Hosting" was way too niche, especially in the age of Facebook Pages, Google Code, cheap cloud instances, etc etc.