And here's the problem: all of them are mutually incompatible in most situations. Even moving .tex documents over different platforms is an enormous pain, and pdflatex/xelatex don't possess the error reporting you need to quickly find which packages are missing on which systems.
Don't get me wrong, I love LaTeX, and I agree that the author misinterprets 'proprietary', but from a user standpoint the problem is the same: old documents are not rendered correctly, and new documents don't work with old compilers. It's a mess.
> all of them are mutually incompatible in most situations.
I haven't compiled against every implementation, but I did just recently rerun a report I create 4 years ago under a different engine. I spent about 20 minutes addressing the new complaints, when I finished that the generated pdf looked exactly the same as the old one - but with up to date data. Try that with html :) Heck, I've had the exact same experience switching C compilers. I do agree with you about the crappy error reporting. There is a reason why the Library of Congress is bundling data with binaries now, this is a very common problem - but in my experience Latex has fared much better than most formats.
Don't get me wrong, I love LaTeX, and I agree that the author misinterprets 'proprietary', but from a user standpoint the problem is the same: old documents are not rendered correctly, and new documents don't work with old compilers. It's a mess.