Yes, we do have customers who use Marketo activity data in their models. However, we primarily suggest separating out behavioral data from fit data - in the same way Marketo uses stars and flames to denote fit vs. engagement.
For more information on Behavioral vs Fit scoring, please see Two approaches to scoring leads – Fit vs. Behavior.
Infer also recently launched stand-alone Behavior Models. For more information, please see Infer’s Behavioral Modeling Takes the Lead.