If it's a big company, it might be good. Be careful though, read the fine print, sometimes the rollover bandwidth is capped, or it expires after a certain amount of time.
It might not be a good idea to do it with a smaller host. Scenario:
You pay for 50 GB / month bandwidth, you use only 10 per month for 12 months, you have 480 GB of Bandwidth rolled over, all of the sudden your site takes off, or you decide to host large files and use 300 GB of that in one month, in more cases the not that will be more then they can handle.