Here's just my 2-cents on this:
- color highlighting improves clarity and readability of code, hence improves clarity and readability of the edited post, makes it easier to read (related post).
- they're not destructive and don't alter original content
- they change wall-of-text code into easier to read code (bullet point 2 from here)
- posts where multiple languages are used can benefit from added syntax highlighting; in fact, I frequently post answers that provide solutions in bash, perl, python, and others, and syntax highlighting helps distinguishing the syntax of each solution.
- merely using tags as dessert's answer suggests can work for single-language posts ( IMHO ), but not in multi-language posts, plus as Zanna pointed out - tags can be removed.
Personally, I see absolutely no problem whatsoever with such edits. In fact, feel free to edit my answers any time just to add syntax high-lighting. I've like 1699 answers by this point, and prettyfying that many is time consuming :)