I have seen some companies for instance httpme had a backup site if their were server problems at httpme.net Sometimes you can have your mainpage on a different server. However that doesn't work for us for instance because our helpdesk system has to be on the same server as our mail so that piping works....I guess it depends really...
It depends upon the situation of course, but often that is a good idea to have some form of backup if things go down. I wouldn't quite go so far as saying that it's something a reseller must do, but it certainly wouldn't hurt if you can afford to do it.
However, you'd want to make sure that it's with a good provider.
I would totally recomend it as most hosting companies have there sites pretty seperate from there customers for that very reason if your customers site go down then you need to be able to support them and at least let them know whats going on. If they see your down too they might start jumping ship.