If you have a custom MMC that you want to use as a "template" for others, MSC files are just XML and can easily be edited in a text editor. Why would you do this instead of using author mode? One word: scripting.
We've just recently given our hub operators the option to manage the AD accounts for their clients for our external-facing systems. I wanted to limit them to their client OU only, so I simply created an MMC with the view I wanted, and replaced all instances of the OU name with a tag which I then ran a script against. I created 60 locked-down, single-view MMCs with less than an hour's actual work (the script also published them all via Citrix so I didn't have to deal with that, too).
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