In the context of a software company, when does it make sense to hire a technical evangelist (also known as advocates now). Big companies have them and I've seen plenty of smaller companies with their own evangelists.
What should I expect from the evangelism path? Is it fair to expect direct sales as a result? meaning, is it fair to compare them to a sales person? Is it worth the cost?
I would expect an evangelist to generate content (blogging, articles, videos, etc), give live demo's and presentations at conferences, communicate with the "community" and act as a type of proxy (acquiring feedback from users/customers)
As an evangelist working at a software company, I'll attempt to answer your question given limited knowledge about your specific case.
You should expect evangelism to result in the the content, demo's and PR you mentioned above. You should not necessarily expect evangelism to translate to sales immediately but it should have a large effect in the long term. Evangelism should function to get your product exposure to the right users. As Guy Kawasaki, in The Macintosh Way, describes early Mac evangelism- the product speaks for itself and the evangelist breaths that confidence and enthusiasm while getting it into the hands of key users. Evangelists can convert influential users of a competing product, representing a single "sale." This type of strategy will translate to disruption of the competition and future sales down the road- not an immediate influx of sales. This can be done through social media or face-to-face relationship building, for example. Or an evangelist can recruit a couple of noteworthy users through a stellar in person demo. As far as whether it is worth the cost to the company and what metrics are used to rate performance- this varies from company to company. Apple benefited greatly from technical evangelism in its startup years and many prominent technology companies today employ evangelists including Microsoft and IBM. For a growing startup evangelism can powerfully supplement marketing, sales and PR efforts.
If you own a software related company/products/services, I would surely suggest you to hire a technical evangelist. Using your financial resources to an evangelist can generate more dedicated and trusted sales than generic SEO and other promotional campaigns where a great percentage of junk and irrelevant traffic is always expected.
Moreover, other than expecting from an evangelist to generate content o your website; engage him on dedicated and targeted resource websites, forums, blogs and also encourage him to ethically promote your company/products by taking part in all discussions on external websites. It is the best use of a technical evangelist that can increase sales and generate true value for your company.