If by sub-mix, you mean using additional buses, that is sort of what the OP was doing, except he kind of took it too far for being practical.
A common thing to do is to route common instruments to a single sub-bus for each instrument group, which is then routed to the master, though additional layering is potentially beneficial, depending on the nature of the bus configuration you end up with.
This is done for a couple of reasons: 1st, grouping like instruments (like 1st violin, 2nd violin, cello, viola), by routing them to a bus (Strings), allows you the flexibility to control individual instruments as needed while also allowing the whole section to be brought up or down. 2nd, 'like' instruments usually have similar kinds of reverb and such applied to them, so you could share a single instance of whatever reverb you wanted to use for the string section. (This saves CPU, and makes changing the effect levels and/or other parameters easier, since doing it once benefits the whole bus).
Folks often route each drum kit piece to its own audio track, then group those tracks by sending things like all the toms to a Toms bus, and all the cymbals to a 'Cymbals' bus, and these buses are then routed to a main 'Drums' bus, which itself is routed to the Master bus.
It's all about grouping 'like' instruments for making it easier to add effects to, to save CPU by reducing the number of effects instances are used, and about controlling volume of grouped tracks with single faders.
If you watch the first part of one of the Sonar videos, on Larger Than Life Drums, they explain this and go through the setup - and the 'why'. Here is a link to that video:
https://www.youtube.com/watch?v=VH9W1MD80kk You can skip ahead to 7:00 into the video, and they will shortly begin adding the extra buses, and explaining how and why they do so.
Bob Bone