Key issue is this: We use a traditional approach of having a "First Offer" and "Last Offer" field (among others, but this is enough to illustrate the issue). "First Offer" is of course written once for the new lead, and "Last Offer" is always overwritten. Of course, the touches between these two is lost. In SFDC, no problem. All touches are also logged in an SFDC campaign object so that you can always fish out everyone who, say, submitted a "request for trial" web form.
In Dynamics, Marketo can't update the Dynamics "campaign" object. (And my reading so far indicates even this may not be helpful because Dynamics campaigns may just be calling lists and not be analogous to SFDC campaigns.)
In the past, this led to the conclusion that we need multiple leads for the same email, one for each touch. Of course, this is wrong for obvious and numerous reasons.
But the question becomes: what's the best practice for multi-touch tracking? (I'm not gracing this with "multi touch ATTRIBUTION," which implies an even higher standard.) Of course, Microsoft's answer is to ditch Marketo and license their marketing package -- a no-go in our case.
One lightweight idea is to create a multi-value picklist, or even just a humongous text field, in Dynamics and just keep appending each new touch. Messy, but at least you can query for individual touches and know you can always track them back to an individual record.
Any other best bets?