It depends on how much downtime you can afford.
If a single hard drive goes down, in RAID1 you are still up and running.
If a single power supply goes down, your server is down until the supply is replaced.
Yes, power supplies in decent data centers tend to last a long time, possibly exceeding server life. It still is a risk though. Additionally, there is a question of how long will the replacement take -- does the datacenter stack the spares, or will it have to be ordered or something.
There are availability estimates for those kinds of things. I would think the datacenter should just provide you an estimate of % of uptime you get for the price, rather than try to involve you in a technical discussion of how to achieve it.
In general, however, if you can afford to have unexpected downtime of server from 30min (if data center stacks the supplies) to up to 3-5 business days(or whatever it takes to get a new one ordered and delivered), then no need for extra power supply. If you'd rather not, consider, if the price is right for having secured yourself against that. In general, extra power supplies shouldn't be very expensive.