Well if you're going to jump at different speakers, then you should know the types of speakers before you dive into something like this.
You have 3 basic types:
Minis, Nearfields, & Mains.
Minis are your "universal reference", meaning that these are usually lower quality mini speakers used to simulate the bandwith of the usual low end mediums out in the world. TV speakers, radios, cheap headphones, outdoor speakers (backyard types or "walmart" types). You might of heard of Auratones, aka "Horror Tones", which serve exactly this purpose. They are shitty on purpose to help check your mixes *occasionally* through these.
Nearfields are your main dogs. They are designed to provide just the right blend of bandwith and EQ response to mix faithfully and neturally. Obviously some are better than others, but they are your main set of ears in any studio.
Mains are good for two reasons. A) they impress the shit out of your customers as they are walking into a studio for the tour. The very reason that you'll usually have music playing on your mains (if they have em) if your checking out a studio for the first time.
B) They are full bandwidth speakers meaning you can switch to your mains to check out the low end response in your mix. Plus it gives you a better idea of the big picture.
They are usually installed in the walls ahead of the console in a wider configuration than your nearfields. Or they can also be installed on custom stands in the same position.
I'd say 90% of the time your probably going to have to rely on your nearfields. When your familiar with them, you tend to trust them more than any other type of speaker. That's why you find some engineers that bring thier own nearfields to sessions, which the studio manager may or may not be more than happy to set the studio nearfields off to the side to set up for you.
I myself first purchased Tannoy Reveal Actives a couple of years ago, and while they are not the best speakers out there, I got so used to them, I know exactly how they react and how to get good sound out of them. I take them everywhere with me.
So I suppose the moral of the story is that if you're comfortable with whatever you got, and you can still create great results, then by all means use them.
Your mono buttom simply converts your mix to mono. It helps you get an idea of how your mix will sound if it ends up on a mono TV or otherwise mediums that compromise the stereo "beauty" of any mix.
For example, engineers on a high class projects level tend to create "narrow" versions of an artists single. Since TV does such a great job of closing your width, they may mix wide sources closer to about 50/50 to help maintain those elements definable.
Not too shabby when you're trying to convince your home audience of falling in love with your guitars over the competition.
Actually, if you want proof of this...try this:
If you have MTV2 and FUSE, try to pick a day to switch back and forth from channels and notice the change in quality. MTV2 translates mixes way better and alot more clearly than Fuse. Which means the system they are broadcasting through is alot better than what Fuse might be able to afford. Who knows, right?
I've also noticed that it might be done in the mastering stage, something that some of the up and comming bands either can't afford, or don't seem to stress upon.
Just for comparison:
Green Day mixes on TV translate well. Fall Out Boy seems to translate well. Avenge Sevenfold translates well. Weezer translates well (naming off some of the shit I've seen a lot of these days).
Yet, My Chemical Romance on TV translates like shit.
So whether it's a marketing tactic to make them sound better next time around, or if they simply can't afford the service on a "new band" budget, is still a mystery to me.