I am not bringing this to light to support PPC. Our agency manages SEO as well — and does a fantastic job of it.
In the end, it is all about the client, their goals and their budget. I prefer to look at my clients short and long term goals. Short term goals are usually about revenue and sales, of which PPC is a good resource. Long term goals tend to be more about strategy and cost efficiencies to which SEO can help accomplish.
Utilizing PPC for top line revenue and incremental growth opportunities is a good solution and one I use with most clients.
Keeping in mind that Google will continue to change algorithms to support their bottom line, investing in SEO is like investing money in a 401K for your business, its money that will gain value over time and remain in your account. Similarly, by investing in SEO you create long-term value for your website that will continue to accrue over time.