Character spacing, kerning, ligatures, and many other aspects of modern typography are all that you give up with type writer fonts. Yes, if you use a shit font like Times New Roman, it will look bad, but if you use a nice San Serif font, it doesn't look as you described (like cursive, comic sans, or another pseudo hand writing font).

Yes, you give up the ability to do ascii art in your code. I find that this doesn't really matter to me or much of the code I use professionally (I can open up some random javascript code off of GitHub and they don't do non indent alignments). I guess if I was programming in C, this might be much more of a problem.