If you want to integrate with Marketo via the REST API, the best practice is to assign a user the API Only role and then create a custom service for use with ReST API. Here's how to create the API only user role.
Admin Permissions Required
1. From My Marketo, click Admin.
2. Under Security, click Users & Roles.
3. Click the Roles tab and then New Role.
4. Enter a Role Name, select the API access Permissions you want to give, and click Create.
Okay, well done! Now let's create an API Only user.