SFDC Formulas are calculated when the record is requested, therefore they don’t change the ‘last modified’ date, so changes don’t automatically sync to Marketo. When another field value on the record changes, the updated value of the formula will be synced. Even if you force a sync between SFDC and Marketo, unless that timestamp is updated, it does nothing.
Here is a fantastic article by Raul Fonseca on Apr 27, 2015 with more details.
A formula field in Salesforce is not reliable for using as a trigger for the reason noted above. I also wouldn’t recommend using a formula field in a segmentation since segmentation changes are based on a data value change. Here is a solution proposed by Edward Masson.
Marketo will not allow you to use formula fields as tokens in emails.
Marketo Formula Fields cannot be sync'd over to RCA. RCA only allows sync'ing over of "native" fields/attributes. That said, if the underlying Formula Field is derived from native fields/attributes that are sync'd over into RCA, you can always create (i.e., recreate) this as a calculated metric within Revenue Explorer.
Here are some additional resources for learning about the Marketo integration with SFDC.