The space in the sonic spectrum that every layer in your track fills is essential to not only mastering, but mixing and even arranging.
If you have two leads playing different parts, in the same frequency range, in mono, they'll walk all over each other. In the stereo spectrum, you can consider the possibility of panning. This is especially true with harmony instruments. Mixing a jazz track with the piano and guitar both centered leaves you with a large possibility that both will not fit in the mix. In stereo, panning one each way leaves them both filling different spaces. In my own metal mixes, there would be no way to have the snare and the rhythm guitars working together if I didn't hard pan the rhythm guitars, leaving space in the middle of the sonic spectrum for the snare to come through.
Not to mention mixing. When you're mixing, spatial placement of each instrument is essential, which is impossible in a mono signal. How are you going to judge the width of the guitar's reverb without hearing the delay between two stereo speakers? Not to mention that any panning effects or panning delays are completely out of the question for getting right, along with your reverbs.
Seriously terrible, terrible advice. It's like telling someone to mold a sculpture, but you can only look at it from one angle.
Edit:
Here's an example of what I'm talking about:
(at 1:10)
There are two main bass synths. The wobble, and the bounce, we'll call them. The wobble is VERY stereo, and the bounce is very centered. Mixing and arranging in mono, this section would NOT work, as these two basses would conflict all over the place. Having the panning during the mixing process allows this section to work. Not only that, but the contrast between the stereo and mono parts are basically what MAKES this whole section interesting.
I don't know if Ghetto Lee considers all processing stages "mastering," which is absolutely wrong, but even if he did, having panning options available even during the arrangement process is essential. And just to clear that up - mastering deals ONLY with global (dealing with the entire song, not individual instruments) effects. If you hire a mastering studio to work on your song, you give them one single audio file per song, the finalized mixes. All the instrument processing has already been done by the mixing studio.