I'm schooled in the 1:1 synch model of M.A. and CRM - matching record in each system for each record in the other.
So, upon inheriting a Marketo/SFDC stack, i'm trying to understand the rationale/argument for - before changing anything - data hygiene and other batch jobs that purge the record from Marketo but leave it in Salesforce.
I mean, I can imagine rationale, but want to hear from others.
It also relates to another issue re: leads in SFDC that are tagged 'NOT synch to Marketo' and then new matching leads introduced to Marketo create duplicates. (long story.)
In the past I have saw organizations do this due to Marketo database limits, let's say you have a 50,000 person database quota in Marketo and SFDC works on the amount of data you use, then it may make sense to remove a person from Marketo but not from SFDC.
What you described in the last part is exactly how I've saw it done before, I don't think I've saw the final solution for it, but what I suggested in the past was to setup something possibly on the SFDC side that checks if a person has 'NOT synch to Marketo' checked but now that person is trying to enter from Marketo and to uncheck the 'NOT synch to Marketo', then let the normal updates happen. I personally think having a field like this in SFDC is crazy and I've only seen error after error once an organization has it and the only reasoning I can see from it is due to DB sizes, I'd look into a DB cleanup and do away with the field altogether in my personal opinion.
Indeed, not synch'ing new leads to SFDC is not such a good idea. People do this because they fear they will overwhelm sales in SFDC with immature leads. The best way to set this up is to create various lead queues in SFDC and assign the leads to the queues based on lead maturity. You will also need to train everyone so that they do not create new lead nor contacts in SFDC without searching SFDC for exiting records (which supposes they can access these leads in the first place).
Once the lead is in Marketo and SFDC as non sync'ed records, it is too late... The duplicate will happen and will need to be merged later.
With regards to sync filtering, I wrote quite a bit here:
@Greg - my situation is actually the opposite - when leads are purged in data hygiene, from Marketo, the record is NOT removed from SFDC at the same time. Doesn't make sense to me. And it leads to the second problem that Frank acknowledged - outsized SFDC database relative to Marketo, and potential for all sorts of errors down the line.