This is a great inquiry, and I have moved it into our Featured Content area!
This is one of the first questions I ask people as well. Obviously, it depends on the number of people and initiatives you are supporting. I started MA at an enterprise size company with 60 people in marketing. Salesforce.com administrators were in a separate department. I focused the majority of my energy on Marketo, but stayed closely linked to the SFDC team. In my current role (at a smaller company) I am the sole SFDC and Marketo administrator. We have 60 SFDC users and 2 Marketo users (6 people in marketing total). Like you, I'm wearing a different hat every day. Getting a second person to start using Marketo was a case I had to make. I pointed out that working on A, B, and C helps directly generate revenue for the company and requires a higher level of expertise. Project D is repeatable, but pops up X times per week when I should be doing A, B, and C. Basically - help me help you.
Remember, your company bought automation to reduce time and resources and probably won't understand the position of an "additional investment in marketing automation." It's an investment in demand generation as a whole - and in you.