I'm confused how you know what level you are outputting when you export audio.  Is the level on your Mains (speakers) a correct indication of the output level?
 
I would not want to make a record without a Master fader I could control (for fade-outs, as just one example) and, as others said, without the option of overall EQ, limiting, etc.  The other thing is that if you ever bring in another engineer or producer to work on your project, it will really mess him/her up to have a non-standard signal routing.
 
I suppose it could work but I think it would be confusing.
 
If you do use a Master bus, be sure and make it the default and make sure EVERYTHING goes to it.  If you have some things going to Master and some bypassing Master and going straight to Mains/Speakers, it will REALLY be confusing.  Trust me—that has happened to me and it made me go nuts until I realized it.  If you mute Master, everything should go silent.

The way I export a mixdown is I use the master buss only as the source.  What is on the master buss is the entire mix. The level is what is shown on that buss. 
 
There may be a misunderstanding. The Main outs, also known as the audio device are shown in the Console View. You have meters and two fader for each stereo out. If you have configured your CV right it should have three panes. One for tracks, one for buses and one for the main outs.  

Here's a "good" reason to bypass the Master output:
 
Say you import a reference track (fully mastered) and wanted to compare the sum of your new tracks (that should be going through the Master Buss) and the reference track. If the reference track goes through the Master Buss it will affected by any processing that the Master has on it (I always have a limiter at least on the Master buss) so will be changed and so its use as a reference is compromised.
If you route the reference track to the direct outs you can A-B the direct outputs and the Master Buss with confidence that each version is what playing what it should.
 
I would like to point out even the bestest Producers use reference tracks so this isn't a lame idea.
 
Grum.

Federal Laws on loudness?

Grumble, I do use reference tracks side by side in projects as you stated. The reference is always set to a direct out bypassing the master. There is one more step that is helpful. Throw frequency analyzer in to the fx bin on your reference track and use one on your master buss as well. ALWAYS, use your ears but the analyzer helps you see how to at least get in the ballpark with your mixes.
I use Ozone 5 to master in X3. It has a feature that allows to play audio from a reference track for a set amount of time,  Ozone analyzes it, and then Ozone sets the EQ for you to closely match the frequency curve of the reference track automatically. Not perfect but gets you real close real quick. For it to work correctly it has to be in the Master buss.

vegas 13 has loudness meters with the legal stuff built in. i now bring my sonar mixes into vegas just to do a reality check - if it's green i'm good to go, red, i gotta bring it down. i can also see if it's too low.

Sixfinger
Federal Laws on loudness?

The CALM Act

so you can apply effects to the whole mix before it hits the "master" out (speakers)

lawp
so you can apply effects to the whole mix before it hits the "master" out (speakers)

The Master out is not the speakers..   You could have (and I do have) more than one set of speakers.
 
On a hardware mixer a standard scenario would be two pairs of speakers (for A/B comparison), a set of headphones and of course a record output.
With Sonar and a sound card your options are limited by the number of outputs on your sound card, but most have at least 4 (2 stereo pairs). 
 
The Master is the point in the chain where all the signals are summed together to produce the final output.  This is what should be sent to disk when you "export".  You send the default output of the Master to your soundcards default speaker output.  If you have extra speakers, add a "post send" to the Master and route it to the next pair of outputs on your sound card.  Use the Soundcard outputs section of the console (very far right side of the console) to mute/unmute the speakers you want to hear.
 
For monitoring of reference tracks:
create a new bus, call it "Reference". Add your favourite limiter in the FX box to protect your speakers from "overs". Send the output to your main speaker output (same as the Master).
Place your reference material in a standard track and route it to the "reference" bus, not the Master.
Quick group the Master and Reference busses with ONE of them muted.   You can now switch between your mix and your reference and back at the press of a single mute button.

Karyn

lawp
so you can apply effects to the whole mix before it hits the "master" out (speakers)

The Master out is not the speakers..   You could have (and I do have) more than one set of speakers.
 

sure, i was using the op's terminology, i also have multiple outs