From stuff I've seen here and elsewhere I think the most important reason for this is congestion at NAPs making it impossible to suck (or shove) lots of bandwidth at anything but your provider's backbone.
In using "NAPs" above, are you just talking about the NSF NAPs or all interconnections? I think it may be an overstatement to say that all NAPs are congested. Perhaps that not what you meant. My experience is that provider backbones are congested at times as well. It is unclear how fast providers can address these types of problems. My experience is that providers often don't have large enough ingress into the NAPs. So, sometimes the NAPs are congested and sometimes the providers have inadequate facilities to make the best use of the NAPs.
I haven't considered yet the maintenance/logistical cost of managing 15 T1s to 6 or 7 providers vs. the "ease" of two frac-T3s to two providers.
Generally for each connection to each provider, you would have to set up BGP. This will have some cost in processing and memory in your border routers. This will also involve dealing with full routing information and maintaining valid filters relating to the the changes that occur in other parts of the Internet. I believe that some providers have people who spend full time just doing this task, so you might have to allocate the same resources to do that. -- Stan | Academ Consulting Services |internet: sob@academ.com Olan | For more info on academ, see this |uucp: {mcsun|amdahl}!academ!sob Barber | URL- http://www.academ.com/academ |Opinions expressed are only mine.