Hi Rachelle,
In short answer to your question: No, you do not need to use SFDC Leads for Marketo to work.
In long answer to your question: Its going to be a pain if you don't.
So before we get into why its going to be such a pain, maybe you can walk through the scenario you're thinking about and why you don't want to use Leads?
To give you a very quick run down on the reasons its going to be a pain:
1. SFDC inherently uses all Web to Lead captures and enters them as Leads.
2. Marketo works similarly to this above process and thus enters their net new records as leads as well.
3. If you use the "Convert to Lead" flow step in Marketo it will also create a new opportunity associated to every net new account, which I'd imagine is not your wish. ALSO, if you're working in B2B, it will create a heck of a mess for you when you have 4 contacts come through from "Acme" company, but they all submit it 4 different ways (Acme, Acme Inc., Acme Company, Acme Pty Ltd.). This would create 4 different accounts with 4 different contacts.
Now are there ways to work around some of these above? Sure...but we're entering a very big world of pain when it comes to process, reporting, and the amount of custom code necessary to accomplish this.
So let's start back at what your scenario is and we can diagnose just how painful it may (or maynot) be. ;).