So I'm getting my dirt basement encapsulated, and the contractor is installing an interior perimeter drain and sump pit/pump. They offer either a single good quality pump, a redundant system with just another pump, and a triple redundant system with a battery backup. I have no reason to believe that I'll seriously flood the basement, since water infiltration has been due to spring thaws and some relatively minor grading issues on one side (recently corrected)... is it worth springing for the double redundancy (~$400 extra) or triple redundancy (~$400 MORE)?
As others have suggested, you should conduct a risk assessment and consider the likelihood and severity of a flood or other fault condition.
Another option I've not seen suggested is to install a float or level switch connected to an alarm sounder/beacon which would indicate a rising level and potentially warning of a pump failure.
A friend of mine did something similar in his wine cellar containing a lot of valuable wine. He ended up with a single pump and an alarm unit detecting room temperature and pump pit high level which would send an SMS over a GSM network to his cell phone.
Most pump failures that I hear about are due to one of several causes. The pump runs on a regular basis due to poor drainage, high water table etc. Eventually it wears out and fails. Usually it fails in the middle of a flood. On other occasions it cycles very infrequently and it corrodes due to it being in a humid/damp sump well and rarely running. The other failure occurs when the pump exceeds its' duty cycle during the one in a hundred year flood. The pump just runs continuously until it fails. I am guessing that the work your contractor is doing is several times the cost of the pump. So you have a sum of money invested already. I would go with the double pump. By the way it is a a real pain to replace the failed pump while the cellar is flooded. As @ Tester101 has stated if time shown that power failures are an issue go with a battery back-up instead. Remember that the battery requires regular servicing and won't run the pump for extended outages. A watered powered unit is only an option if you have municipal water.
If you frequently have power outages, a redundant system could well be worth the cost. This is especially true since power outages are common during storms, which could be a cause of rising waters.
If you do get a redundant system, you'll want to get one that doesn't rely on the same power source as the primary. So you'll want a battery, or water powered system.