Well, if you're willing to throw yourself into the role,
somebody has to deal with marketing, PR, strategy, sales, business development, bookkeeping, regulatory compliance, making coffee, taking out the trash, etc.
So yeah.. there are a LOT of places for a non-technical co-founder to add value. Since most techies aren't very marketing-savvy, I'd suggest you go buy every marketing book you can get your hands on, and do a deep dive.
OK, maybe not literally every marketing book. But if you're interested, here are a few items I'd recommend to a budding would-be marketer.
Permission Marketing - Seth Godin
All Marketers Are Liars - Seth Godin
Purple Cow - Seth Godin
Unleashing The Ideavirus - Seth Godin
Positioning - Jack Trout and Al Ries
Differentiate or Die - Jack Trout
The 22 Immutable Laws of Marketing - Al Ries
Repositioning - Jack Trout
Marketing Warfare - Al Ries
The 22 Immutable Laws of Branding - Al Ries and Laura Ries
The Discipline of Market Leaders - Michael Treacy and Fred Wiersema
Blue Ocean Strategy - W. Chan Kim and Renee Mauborgne
Trust Me, I'm Lying - Confessions of a Media Manipulator - Ryan Holiday
Crossing the Chasm - Geoffrey Moore
If this is a B2B play, I'd read Steve Blank's The Four Steps to the Epiphany first though. Otherwise, I'd suggest starting with The Discipline of Market Leaders and then go to All Marketers Are Liars.
What kind of duties would you be doing in a role like this? Writing web copy; writing press releases; writing whitepapers; writing blog posts; recording screencasts & demo videos; setting up mailing lists; running the Web, Twitter, Facebook, G+, Pinterest, Tumblr, Youtube, etc., presence; talking to potential customers and soliciting feedback; developing a marketing strategy, PR strategy, etc; working with your cofounders to choose a "value discipline" (See The Discipline of Market Leaders) and putting an operating model in place to fulfill that discipline; possibly talking to potential investors; talking to potential partners, doing business development; and SOOOOOOO very much more.