That's pretty much it.
On most amps, you adjust your amp to suit your tubes.
On a Mesa, you select your tubes to suit your amp.
So, when you order tubes you tell them it's for a (insert model here) amp, and they send tubes that they know fall within that amps particular bias range.
The drawback is that you can't bias your amp hot or cold to adjust your tone. The plus is that you can choose tubes that are hot or cold to adjust your tone. It's also a bonus that if a tube dies on you at an inoppertune moment, you can pop another tube of the same grade and carry on with the show.
Sorry for the rambling, but I'm bored