The degree of prepending you show is probably more for Sprint's
benefit than anyone else's. 1793, 1239, and 1790 are all Sprint AS's.
If you're lucky, you can find someone familiar with Sprint's
confederated routing scheme to explain the details. AS3505
belongs to VNet, who uses both Sprint and Gridnet as providers. Prepending
allows them (VNet) to effectively designate Gridnet as their primary
(all things being equal, better--shorter--AS path wins).
As for hold-down routes (to Null0) and damping, yes, they're both
good and generally practiced ideas. As to whether and to what extent
Sprint uses either or both of them, you will have to take that up
with them: noc@sprint.net or 1-800-230-5108
jim