As I was the principle person responsible for setting up the network during it's birth, I am going to go out on a limb and defend taking default routes. Routers/switches with large amounts of memory for routing are not cheap, years and years ago when the routing table was ~200k routes (with /24's mixed in there), a 6500 series with a decent sup and nice backplane bandwidth was about $40k plus (direct from my old vendor).
On the other hand, a smaller, faster switch (high PPS) was cheaper and could do more for less money (trunking, hsrp, etc). We went the cheaper route. While I /wanted/ a fully loaded 6506 with sup720 (they were $25k new), but we could not justify the cost JUST for full bgp routes. 95% of dedicated customers did not care about if we had full BGP or not, as long as their service worked (and didn't lag).
Back in the 90s when I worked for AboveNet, the motto at IAD4 was 'Keep it simple', which is exactly what I used when I designed the existing network. OSPF with eBGP (default routes), we used communities to alter our localpref in some cases, though. It worked.
One thing I did NOT agree with is mixing vendor equipment, it's nice to do layer2 on different vendors (most of the time) but firmware bugs, broken RFC agreements made it difficult. I despised foundry's gear as being total shit made by people who wear leafs as shoes.
It's simple numbers at the end of the day, the needs of the many (simple setup, default routes) outweigh the needs of the few (full blown routing, MPLS, et al)