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