The argument here is probably that it may be rather difficult to choose such a private AS number, if you have lots and lots of customers, that it would never clash with their customers' (depending on how it's being used) internal usage.
Uhm, you are saying that there would be singlehomed customers (let's call them A) of a ISP, that would need BGP to talk to the provider. Customer A then in turn would have single-homed customers that would need to talk BGP to them? This sounds like a very teoretical excersie to me, but maybe people are building their networks liek this. In that case I suggest they figure out how to deal with this...:)
In general, I don't think this kind of AS number use is appropriate in a bigger scale.
Correct. And the Internet is becoming pretty large scale. - kurtis -