MSI is optional. It is not required to sync the systems.
Maybe I should be clearer - in the past it may have mattered to add that, even though you did not have to display the MSI data to users.
AFAIK, you can safely install the MLM package and sync the systems without MSI. You could also install MSI and not display the items to any users.
There is no more MLM package. if you need some sync fields, you will have to create them using the new nomenclature.
And yes, the MSI package is optional.
Without MSI, you will get a full sync between Marketo and SFDC. What you will not have are the MSI component in the screen layouts, nor the "stars and flames", neither the Marketo MSI tab when you can review your leads and the "Best bets".