As Chopper said, a striped data format will make it slightly less convenient to recover data from your drives. If you're RMAing the drives, you absolutely cannot count on any sort of wipe operation to do much good, since the most likely failure mode prevents you from reliably writing to some sectors, and the other failure mode prevents you from verifying that you have written to them.
Consider your adversary. A professional team will have no problem recovering a LOT of data from any unwiped raid drive, even if the heads are crashed. The company doing the refurbishment work is unlikely to be interested in your data, and they do zero drives and do a read/write on each sector before sending them out to another customer, meaning that only a very determined adversary would be able to recover data after that operation.
In industries where this is a serious consideration, drives are simply destroyed and not RMA'd. Operational security is considered far more valuable than the money saved by returning the drives. Large organizations negotiate discounts in exchange for agreeing not to utilize the standard warranty process, and either simply report failure rates for replacement, or absorb failure costs, depending on the agreement.
If you absolutely must RMA the drives and retain data security, your only option is full disk encryption. This will impact your performance significantly, and require you to enter a passphrase by hand every time the machine boots.
If you can budget any money for this, buy better quality drives. If you really expect a 50% failure rate over the expected life of your drive array, I'd wager you're not using enterprise class hardware or are planning to keep it far beyond its useful lifespan. Buy enterprise grade drives and replace your drives and arrays with newer hardware within a reasonable timeframe.
Pragmatically, if you simply want the most convenient solution without spending further money or sacrificing performance, use a striped data format to make it less convenient to recover data, and use a disk wiping utility like DBAN to remove as much data as possible from failing disks before you RMA them. Depending on your security needs, drives with crashed heads that won't wipe significant data should be destroyed professionally rather than RMA'd.
If you want to read an interesting report about failure rates of consumer-grade drives in a very large organization, Google's report on that is quite interesting: