I offer many services to clients and sometimes find myself being hired for one particular one while other providers supply the rest.
The problem with being hired for just SEO is people do not realise what other skills you have. I work a lot with retail and start ups.
The issue with being seen as the SEO guy is you can be brought in much later in the process. I had a client who who was manufacturing a new range of products. It was a great idea and they had an small team of suppliers all providing parts and services. I asked what the price of the product was, which sounded great until I also asked how much it cost to produce.
There was very little margin left for profit and if one or more of the suppliers had put up their price, or postage prices increased, it would leave nothing. They also had not taken into account how much budget would be required for marketing online. Once you launch a new product online it is very hard to increase the price later on.
Having a healthy margin means you can offer bundles, special offers and still make a profit.
It also means if suppliers do increase prices you have room to breath and innovate.