SEO is just a small part of the whole internet marketing industry, there are alot of other marketing ideas that can be and should be implemented along with
SEO to get proper results.
You should not keep all your eggs in one basket
SEO).
Like News / Press Releases, Affiliate marketing, Newsletter development, Mailing lists (not spamming) but geneuine content oriented and many other up-comming technologies like RSS, Podcasting etc.
SEO is a big part of the industry as with Pay Per click programs, you only get results as long as you are paying, the moment you stop paying, you are not there, but
seo rankings remain for a considerable time.
So
SEO is very attractive to companies of all sizes.
There is another twist to the story,
SEO Versus PPC.
As most of the big search engines are earning money through PPC so it is in their interest, if there is in-consistency in
SEO results, as the only alternative would be PPC. And PPC is their bread and butter, hence the regular changes in algorithms facilitates the conversion of
SEO budgets into PPC.
This is just my opionion. You may convince me otherwise.