ads would be the end of netflix sub for me. I hate hulu for the ads, so much so that I have firefox extensions to skip them. I think ads on netflix would be a detractor, and since I already pay $10 per month, I think ads would be unnecessary for their continued success.
Just wait until Netflix needs to renegotiate major content deals. You won't be seeing $10/mth without commercials much longer. Either the newer content will disappear, or you'll pay more...monthly sub rate and/or advertisements. Throw in some caps and metered billing from ISPs and Netflix will not be the money saver that it is now for many.