http://en.wikipedia.org/wiki/Failure_rate
The person that wrote the Wikipedia article you referenced read the same mythology you did; repeating it doesn't make it true. The plural of anecdote is not fact.
Think about it yourself for a moment. If two cars are traveling 50mph, does that make their average speed 25mph (50/2)? Applying a divisor to a failure rate based on the number of devices is nonsensical.
Perhaps also call up LSI and Adaptec, who use the same formula in their documentation.
http://storageadvisors.adaptec.com/2005/11/01/raid-reliabili...
But what do they know, they only build raid controllers...
Erm wait, didn't I just suggest the exact opposite?
If you really think everyone has been wrong about this all the time then please, by all means, correct wikipedia or write a blog post about the matter.
This "false" formula has been out there for quite some time and you find it in pretty much every write-up on the topic, including those from RAID-vendors who (I'd hope) have spent some thought on these things.
On the flip-side I haven't found a single source to support your thesis. Thus I'd say the burden of proof is on you.