I currently have two main machines (a desktop and a laptop) and I explicitly treat them as "main" machines. They hold the same data (synced using scripts around git and unison), they both run Arch and both run a virtually identical setup. It used to be three machines (slow and quiet laptop for most of the work, fast and loud desktop for gaming and a netbook for mobile stuff) and I built all those syncing tools during that time. I don't use any cloud services for syncing.

It's important to me that those machines can work independently from another. This is also some kind of "implicit backup": Since almost all (!) of my data is synced to the partner machine on a daily basis, very little data would be lost if one machine died. (I still do additional backups of both machines, though.)

Granted, I spend most of the time in front of my desktop computer, simply because it's more comfortable. That doesn't downgrade the laptop to a "secondary" machine, though. It's my "morning machine" (nice term :)), I keep it around when travelling or visiting someone. The point is, it's not a "burden" to use the laptop. It can do anything my desktop can do. No data is missing. This is an enormous luxury, in my opinion.

It took me a lot of time to get this setup up and running. I only started using bazaar in 2007, git in 2008 and unison in 2010. All the stuff that had been created prior to that had to be sorted out and moved around and merged and cleaned up... To some degree, this is still an ongoing process. But it's worth it. I doesn't feel like "scattering files around" anymore (it surely did at the beginning), quite the contrary: If stuff happens to be NOT covered by git or unison, it feels like that data is "lost" or "orphaned". :)

Edit: I didn't mention other devices like my Pi or the Pentium 133 because I feel they're out of scope. They serve very special purposes and are unique. There's no "main" or "secondary" here.