Having two firewalls is as you say purely a matter of redundancy. If you are running active/standby then there is no other benefit to be had other than if a firewall or one or more of it's interfaces fails you have a standby firewall.
Redundancy is a good thing to have in a network but there is obviously a cost associated with this and it is always a tradeoff in terms of design between the two. The company may not need redundancy ie. internet access is not vital although this becoming less common as the internet becomes more important to all companies.
If though you were a web hosting company, for example, you would definitely want that sort of redundancy (and probably a lot more).
Something else to consider is the redundancy of other devices in the network eg. if you have two firewalls but they both connect to the same switch and the switch fails you have lost both of your firewalls. The only real redundancy you get here is if the active firewall interface connecting to the switch fails.
Similarly if you have a single router and then redundant firewalls again you have created a single point of failure.
Redundancy is expensive when you look at it like this ie. two switches, two firewalls, two routers etc. You can move to it bit by bit but until you have the full solution you are not really getting the full benefit. And that does not cover power, which you mention in your post ie. if you have fully redundant network kit then you would want each device within a redundant pair on a completely different power supply.
Again it is difficult to give a definitive answer because as i say it depends on the compnay requirements, the cost of losing connectivlty, the amount of money the company is prepared to spend etc.
Jon
