Hi Chris,
Or, you just don't want to send traffic through Bill Manning's ASN because you dislike his hawiian T-Shirt Policy? There are probably a few hundred reasosn why you'd avoid an ASN... In general though I'd think that like Michel said: "It's a pain and its doing something that bgp should do for you without lots of messing about"
That's why i explicitly asked for some "social/political/etc." reasons where an ISP may not want his traffic to traverse some particular AS number(s). Something which is beyond BGP to determine as of now ! :-) I believe with the responses that i received both on the list and offline, that it is indeed quite normal for ISPs to filter routes based on the AS Paths for 'other' reasons. Reasons, quite beyond BGP as a protocol to handle! And this can happen, when an ISP doesnt want its traffic to traverse some AS(es). Thanks, Tulip