TrffcSndrs brings up some great points.
Both SEO and PPC service models have their advantages and disadvantages.
PPC is easy - you need not alot of knowledge and skills to develop fruition. A great advantage to someone who does not have the time to learn optimization or the upfront to paid for it.
PPC is direct market access - limited competition for eyeballs
PPC is exceptional for short term specials, holiday promotion, etc.
PPC has no middleman to rely on for performance
PPC highly adaptive to market trends (e.g. last week "widget" was hot - this week "widgets" is)
PPC is near-real time no monthly waiting to produce results
PPC is highly cost effective (e.g. can produce 3000% return on investment)
PPC is highly effective in controlling market saturation (e.g. - out of inventory - immediate ads drop). Powering down your web site in other model can have long lasting negative effects. Noting that telling already paid customers they need to wait 3 week is very bad.
Paying for the entry when you don't have the necessary products for the buyer (e.g. using "software" when the only software you have is a Windows 3.1) and few qualified visitors
Using informative phrases that offer signifcant clicks but possibly 95% of click are "free information seeking" only and never buy.
Quick loss of placement in favor of someone else willing to pay more than your budget allows.
Stuck with the model - when the search engine adapts to a new scheme you "must" meet/match their Terms of Service and/or changes.
If PPC is the only service used - you will never be an authority web site on anything - and authority defines - market trust, and your credibility.
PPC never allows the web site owner to truly fault find problem areas of the web site.
PPC favors big business with large marketing budgets.
SEO has incredible ROI. Mostly pay per work - thus per thousand conversion cost per unit sold is less. PPC this goes up and up.
SEO is ongoing designing. Ranked performance is a measurement in itself of how good the site is (as a whole).
SEO is highly adaptive to market diversity. (e.g. a single user that conducts a deep search finds you, identifying new opportunities to expand. (with PPC your stuck with your own knowledge of markets, and/or the engine's ability to distinguish commonality). Five words on the page can match perfectly to a query not ever considered.
SEO is developing authority status - which is directly associated with brand. Many believe branding is reserved for Microsoft or IBM but brand equates to quality - thus no brand potential you have no quality recognition.
SEO in the long-term in more cost effective - as time passes cost reduces. With PPC cost only go up not down.
SEO is more adaptive to search engine changes (medium changes). An algo tweak - can be quickly observed and countered.
SEO provides growth and theming - back to design - to rank high a site must be developed with the market in mind with lots of informative content which develops links, which develop enhanced preformance which develops more links, and so on.
SEO puts the web site owner in the drivers seat - PPC puts the engine in the driver seat.
SEO has a better chance of delivering return customers. You must really think about what the market wants. With PPC it's more what the company wants.
SEO normally has a relative long time to market. (can not be achieved in a day, a week and sometimes months).
SEO is a process of adjustments (tweaking) and more dependent on competitive landscape.
SEO normally requires a knowledgeable third party who may or not be what they appear (e.g. not very good).
SEO can be a disastrous overnight. (penalties, bans) which really sucks since the time to market was long.
SEO can be very expensive upfront.
SEO has a long learning curve and with many abrupt changes where the site's owner can drop quickly from page 1 to page 100.
SEO effects are not immediately seen thus something completed 3 months ago can be mistaken for changes made last week.
And lots and lots and lots of mis-information available that can have a site owner going in the wrong direction - where recover is near impossible (have a new client that did this - then abrupt changes directions, and now the site makes no sense).
I tried to be as unbiased as possible - in the end using both has many merits.