I consider everything that is pushed into SFDC an MQL. After that point SQLs and other measurements are done by lead status/conversion/opportunity within SFDC.
It just makes life easier to restrict the CRM to only MQLs, so that way if I want to pull a report in Marketo of emails sent for something and I want to know how many MQLs I hit, I can filter it by "SFDC Created Date = Not Empty"