One of my customers just had a T1 with a Tier 1 provider down for over a week. The SLA can promise whatever but when it takes a police detail, etc. to get under the street for repairs, you're on phone company time. Single points of failure are still single points of failure regardless of contractual support agreements. I deal with a fair number of MPLS circuits both domestically and internationally and for any site that's even remotely important they end up getting a secondary circuit of some sort (often a cheap Internet line to both offload Internet traffic from the WAN and to act as backup WAN connectivity via IPsec VPN.)
For systems, several redundant, lower performance, inexpensive components are often "better" (e.g. RAID, load balanced x86 webservers, etc.) Sometimes due to various requirements you have to go with a big expensive, high performance system (for example Oracle's licensing is such that for many mid-size companies RAC isn't an option so they end up with one big expensive Sun or Linux box) Likewise with Leased lines - if you're somewhere where a leased line is the only option, or are dealing with latency sensitive traffic you have to suck it up and pay for T1/MPLS/etc. For many traffic profiles however having two lower reliability/SLA connections will provide both better performance (business cable/DSL/MetroE Internet is generally much higher bandwidth than a T1 at half or a third the price) and higher uptime. It's also nice to have a secondary connection on different media since if you're somewhere where the local loop infrastructure sucks, having two T1s from different providers may not provide as much redundancy as you think.
For larger companies, this probably doesn't hold true (having to manage different providers gets to be a hassle with more than a handful of sites, and larger customers get better pricing and more responsive support regardless of SLA) but for small and mid-size companies I think that it's time to question the value of leased lines as dogma.