It's not really highlighting but I recently switched to a proportional font after reading some recommendations and found it noticably better.
Before switching my main fear was things would not line up properly but after trying it I realised it worked fine without any apparent disadvantage. Tabs (or spaces) line up the blocks and words that repeat down the page line up because they are made-up of the same letters. My fear was imaginary because I somehow thought I would need words to line up if they were different . This is not the case.
One thing it prevents is aligning words that are in the middle of expressions, not at the left. E.g. In a fixed width font you can do:
string firstName = "John";
string lastName = "Smith";
int age = 30;
Whereas you can't do that with a proportional font. But that practice seems rare these days. Perhaps more of a C or assembly language thing.
I recommend trying it anyway. The internet runs on proportional fonts and so it makes sense to use them, as long as they don't have any disadvantages. When it was recommended to me they claimed it would only take a few days to get used to and that turned out to be true. I find Arial 11 point to be a good choice in Visual Studio.
