Do I really need a dedicated SFDC license for sync?

We use a shared user for one of our SFDC integrations but only because that integration must have unrestricted access to everything in SFDC so it makes no difference (plus they never take ownership of anything via the API).

I wouldn't even think of using a shared user for Marketo for the reasons the other guys mentioned. 

To my mind, the main problem with using a personal SFDC account as the sync user is if you ever want to restrict your sync in any way. 

As Edward mentioned, restricting field visibility is one example. Maybe you have a field that makes no difference to Marketo but changes often, perhaps via a workflow rule. This will constantly be triggering a sync, increasing API calls. This won't make much difference in the beginning, but it can as you scale. 

Another issue -- what if you want to prevent some records from synching over to Marketo entirely, example, leads or contacts with no email address? Typically this is done via restricting sharing rules on the sync user. But if that user is your admin user that you use for other purposes, then you have a problem. 

Lastly, there is the password issue. Salesforce will occassionally expire your password and then break your sync unless you update the credentials in Marketo quickly. If you have a dedicated sync user you could potentially set them as an API-only user and tell Salesforce not to expire the password (some businesses may be comfortable with that, others not, but at least it's an option). You may not want to do that with a personal account though. 

All that being said, when I worked at a small company we used my personal account for a while as the sync user and it was fine. But eventually it did make sense to have a dedicated account for the reasons above. 

Justin Norris | Perkuto

Thanks Edward - so it's a performance issue? We're not a huge company so don't have a huge amount of data.  My management doesn't want to purchase an additional license without any justifiable business reason, so unless someone explains what's the purpose of the dedicated license I have to manage this with existing licenses. 

I still don't see why we can't use MY license if I don't have any leads or opportunities?

It's best not to use YOUR login account. Rather set up another SFDC users specifcally for Marketo Sync. Thats your user. Then you can limit that user access in SFDC. You may not need to sync every single field etc accoss to Marketo.
This will help with performance depending on the size of you data in SFDC.