Not using SFDC Leads

It is pretty common to only use Contacts, especially for companies that already know exactly who they are selling to, and who have most of those people in their CRM already. 

Just keep in mind that net-new Leads in Marketo will be synced to CRM as Leads, and will need to be converted to become Contacts. I've seen some customers load new names in CRM as Contacts, which then syncs down to Marketo (rather than upload names into Marketo first). Alternatively, partners like Ringlead could push net-new Leads into SFDC as Contacts, doing fuzzy matching to find the correct Account. 

PS. when I say "CRM", I mean any CRM system (SFDC, Dynamics, SAP, etc.), because this question applies to all of them

At my previous company we started off using mostly contacts then decided we may use leads bc that's how Marketo imported them. It became very unorganized.

Depending on the type of company you're with I think it could work but you should have a process developed and shared first.

Developing a process as Kim mentioned is a good advice. There's some confusion now in our Marketing department because they're not clear on how a lead that comes from Marketo syncs over to SFDC.  We sync from SFDC to Marketo as contacts. But when we sync leads from Marketo to SFDC  it syncs over as a lead.  Then we run into problems with duplicates because one record is a lead and another is a contact. Or when someone is inactive in SFDC/Marketo and if they are in a list for import in Marketo, I've seen duplicates and when I try to dedupe I get an error message in the form of one is active and one is inactive.