Hmm, interesting idea with IP-per-customer... but it would be a lot of manual management. Plus when IP's are moved between servers, even after network service restart... somehow the ARPing doesn't broadcast well at times...and renders the IP non-routable for sometime.
Eventually, IP does route after sometime...but it means downtime. Secondly, allocation of individual IP's to machines is a sure management hell... right now we just mark IP blocks....then each IP will have to tracked individually.
The client-per-server part you are right...but when selling semi-dedi accounts the marketing feel-good thing kicks in bigtime. If someone's going to pay $60 he's going to ask for 2 clear answers...
1] How many clients per server.
2] How many sites per server.
Telling a potential customer "it depends upon load" is a sure way to drop the sale
It works for Shared/Reseller (and we ALSO do the same) ...but will drop sales for perfomance-oriented accounts.