RE: IGP metrics on WAN links

MPLS allows you to do the best of both worlds. You can set your TE metrics to the latency/route-miles and then build bandwidth constrained LSPs from place to place and they will take the least latent, uncongested path that they fit on.

Similar things can be done with ATM routing costs, and contracts on spvcs.
Both of these are considerably more work, but it gets traffic flows on good paths very well.

There is at least one large ISP doing metrics in that manner.