Yosemite is NOT on the Systems Requirements list for Dorico and so obviously it’s not recommended. So let me start with a caution that the following is just my own personal experience and may not work for you. However, I did install Dorico on a MacBook running Yosemite and so far have had no problems – not a single one.
I’m not recommending that you try this at home, but I’ll give you my reasoning and let you make up your own mind given your own circumstances. Most importantly, I have a LOT of different audio software and try out more every week, and much of it doesn’t support Sierra. I plan to move to Sierra as soon as all of my most critical software is supported, and I just could not wait to try Dorico. I have a technical background as well as a musical one, so I’m not too worried about crashes (and have not had any) or other anomalies that I expect to pop up. None have yet.
Ease of Use
I thought it would take me longer to run my tests but the most important thing I can say about Dorico is that I am already writing real music with it! Even though its interface is much different from Sibelius, Finale, or MuseScore I find that it has its own logic that must make more sense to my sub-conscious than my conscious mind because I’d have a hard time explaining it briefly, and yet my fingers are already finding the right keystrokes to get the music in my head into Dorico. Of course you can always use the mouse if you forget a keyboard shortcut.
My “training” consists of the 7 tutorials on this page, as well as Daniel Spreadbury’s demo at the unveiling. If you are still on the fence, watch the brief tutorials and notice how easy it is to create a score (even faster using only the computer keyboard, although Daniel shows how simple it is using a mouse) and how amazingly flexible Dorico is in virtually every aspect.
Notice that I am skipping over El Capitan (OS X 10.11) and going straight to Sierra (10.12) because of Sierra’s security features. Obviously I can’t speak about Sierra from experience, but it does make sense to go to the latest OS with better security — once the “bleeding edge” or “early” adopters find the worst of the bugs, and the software companies catch up.
In summary, when you move to Dorico is the question.