My experence to your question was for another instance but around not having to recreate every campaign again.
We needed another instance of Marketo for one of our business units. We needed to extract 3 million records and upload them to the new instance. This was going to take months due to their speed of export/import CSV files. This was not going to work for us.
Also recreate campaigns etc. The amout of work to create everything again in the new instance was too much to bear, we pushed for an alternative and Marketo finally gave in and told us they can COPY the complete instance.
Every single thing came over, took a week or so of copying but it was every single item. All data was there and we just needed to delete the records we did not want, which was faster to delete than to upload the ones we want to keep. Only thing about the copy proces that all campaigns are turned off. We only turned back on the current year and left everything off (about a few years worth of campaigns).
Not sure why they don't talk more about copying over instances but it work perfectly.
We also had an issue some years back with our Marketo Sandbox. Some how our support team deleted our SFDC sandbox which was connected to the MArketo Sandbox. We had to start again and be given a new Marketo sandbox to connect to our new SFDC sandbox. To this day we still get issue from Marketo support when I ask for things in our Marketo sandbox, they always get confuesd and ask me which sandbox... so they still keep the old one on record somewhere.
Hope this is helpful.