I kind of like the idea as an option only for relatively small databases. But I have to stop and think about it and why the option doesn't already exist. Most of the time whenever I needed to restore a database, it was because the that database had some how become corrupted with bad data, in which case, why would I want to have an option to keep it around in case the restore fails or is canceled? On the other hand why would I want to restore a perfectly good database.

An answer to the second question maybe, to eliminate a large accumulation of temporary or test data and reset it back to a baseline, then this option could be a good thing. Of course a transactional script could do the same thing unless the temp/test data had overwritten the data I want kept.

Possibly the reason that it is not already an option is because it would rarely be needed. Then again it does seem that it is those rare cases that always seem to rise up and bite us in the butt.