I was watching an interview - I believe it was Martha Stewart - and she was asked if it really made sense to watch these people for 13 weeks before picking the last person for the job. Interestingly enough, and this makes sense in hindsight, she actually preferred the concept of watching a person work over a long period of time before choosing them for the job.

Now, obviously this wouldn't be necessary for all positions, and is logistically impossible for large companies, but to see someone's work habitats and behavior over time before deciding to hire them would certainly tell you a lot more than an interview or two and a few letters of recommendations.