Beyond spider accesibility issues, an SEO provides no value to a search engine whatsoever. In fact, they can and do prevent the search engines customers' buying sponsored listings. In this respect, SEOs are a commercial risk for a search engine.
I would have to strongly disagree. There have been many clients throughout the years that when they first came to us, their sites were a horrid mess. They would have great information, products or services to offer but because of the lack of proper SEO, a search engine could not decipher what the site was about. I have seen sites that had the same title and meta description tags in every page or worse, no title and meta description tags, body content that was a bunch or sales hype instead of descriptive and relevant copy, etc,. etc., etc.
We corrected these problems so that people could then find the products, services or information they had to offer.
SEO is not always about helping someone rank higher than their competitor but rather helping site owners make sure that what they have to offer can be found.
Unfortunately there are a massive amount of sites out there that are poorly optimized and flood the SERPs with irrelevant information. I was sitting down with a potential client today who is offering a regional service and in trying to pull up something relevant to what they had to offer, I found nothing but irrelevant results (that was on Goolge mind you). Therefore the task of optimizing their site will be easy and once it begins to show in place of the garbage I found today, it will be a relevant result to an end user.
As long as SEOs are helping people to make their sites easier to find by optimizing all of the elements we know to optimize, then it helps SEs because they have relevant results to display to their end users.
As for sponsored listings, any smart SEO will always, always recommend a client spend money on sponsored listings in conjunction with traditional SEO. I know we do. It's just that not every client has the budget. However, if a search engine does not have relevant results because they have a bunch of crap that they cannot identify what it is, then why would anyone use them and why then would anyone want to advertise with them?
It is the spamming SEOs that Ses do not need but the ones who practice ethical SEO, they compliment the SEs indices IMO.