Many of the bigger companies do take regular, sometimes even daily backups. The first issue with the viability of this would be how easy it is to do the backups as a reseller without taking a huge tow on your data transfer usage. The second is the value, as seen by the end users. If they use databases, regular backups become more important, and with the right wording, you can convince them that this is a huge benefit.
Another thing that could differentiate your offer from most others, would be actual direct access of the customer, to the backups that have been made. Make sure you don't make it seem like you guarantee availability of data. Backups or no backups, there's always a very unfortunate scenario where data could go missing.