want to know a few large isps that run this setup?
http://stats.ptptech.com they usally run at 4gbps.. oh and look at orange fiber they are running 25gbps and they are running their routing on 6500's.
the reason why people do this route is usally because a true port cost on a real router such as a juniper is 8-9k PER PORT for a gige. Not to mention juniper only made a handful of 10Gb pics and sold for ~100k each and are no longer supported by Juniper. I'd gladly spend 15k on a sup720 card and another 7k on a 16port gbic card.