Elec info needed 20A circuits

Think of it this way: multi-wire branch circuits are primarily a labor-saving technique when you need two circuits in the same place on the other side of the house. The time saved can be quite significant in more sophisticated wiring jobs.
It often doesn't save much money in materials, because due to the way demand/supply/volume works out, two lengths of, say, 12/2 often cost less than one length of 12/3, despite it needing two copper conductors _more_.
If your electrician is on hourly rates, it's probably better to go with the 12/3 if circumstances permit.
The only real drawback is that people (and even some electricians!) aren't very familiar with them. A secondary problem can be you might not be able to figure out which circuit is overloading if the breaker trips. But there are ways to diagnose that. Another issue is if the installer is sloppy, lazy and/or doesn't know about the neutral pigtailing requirements.
I use them occasionally when it makes good sense.
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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