I also concur with Ed and Josh on this. Especially, in the absence of better data management tools, the burden associated with managing a sandbox can be really painful and using a dedicated WS/Partition is much much simpler.
Just a point: you can ask support to connect your 2 Marketo instances, so that at least you will be able to exchange programs, at least under certain conditions.
One of the things that is really painful is when your SFDC UAT is being refreshed to reflect a dev instance... because reconnecting the SFDC sandbox and the Marketo sandbox become really painful, data model might have changed on SFDC, fields disappeared, other suddenly appeared, etc...
Now, some large companies just do not consider it possible to go live with any software before they have intensly tested on a sandbox, and Marketo impossibility to change SFDC connection may make the Marketo sandbox mandatory in such environments.
We have a sandbox that's almost never used, except sometimes when our Development team is building something new and since they don't normally work in production they are basically afraid of working in production. (It freaks them out when I go in and just start making random changes to stuff, and they're always reminding me that I'm in production....)
But, if you do move forward with it, one thing that we encountered during one of our renewals was they tried to tell us we had 50% more people in our database than we actually did.. In the end it turns out they were counting the records in our sandbox towards our total! We put an end to that very quickly, but it's something to watch out for.