I know Mathematica like the back of my hand, but I do not know a speck of $\LaTeX$ or $\TeX$. With regard to mathematical typesetting, is there something significant I can do in $\LaTeX$/$\TeX$ that I can't do in Mathematica? The Mathematica interface for typesetting seems much more intuitive and I already know it. However, when it comes to mathematical typesetting, I know $\LaTeX$ sets the standard.
By the way: I know Mathematica is expensive and $\LaTeX$ is free, but I already own Mathematica and use it for many other things, so will always have a copy of it. So I mean my question to be independent of price.

$\LaTeX$ is not only free like in beer, but free like in speech. There are enormous advantages to using open source standards.
Right now you like Mathematica and are happy with its software. Suppose that five years from now the company decides to put out a new version and break backwards-compatibility. Or the company goes out of business. And suddenly the files you have become useless, because they're stored in a proprietary format that nobody can read.
$\LaTeX$ has a huge community of people contributing code to the standard. Thousands of user-written packages to do specific things that the base code doesn't. Mathematica only has the Mathematica staff.
Response added to the discussion that appeared suddenly below:
Microsoft Word is great and has huge market share. Before Word there was Word Perfect, that was great and had huge market share. Before Word Perfect there was Word Star, that was great and had huge market share.
You use closed-source software at your own risk. I use Word myself on occasion (it's installed at work, and people send me Word files), but I would never invest thousands of hours of my time in developing expertise in software that will very likely be obsolete in my lifetime. Open-source standards might become dormant, but they are never dead, because the source is still available and someone can bring them back to life.
I'm not Stallman; I'm not suggesting that closed-source is evil and we should do open-source or nothing. However, using closed-source software is a legitimate risk, and one specific advantage $\LaTeX$ has over Mathematica. In a case of mature, full-featured alternatives (such as in this case) I always prefer the open source one.