This is what e.g. many Haskell developers do, the issue is that this will usually fuck up alignment (because the base font does not contain the non-ascii symbols) and generate different alignments between different viewing contexts. It also leads to odd issues where deleting a character just changes it and doesn't move the cursor at all.

Ligatures are a time-honored way to improve the look and feel of combinations of characters, that's basically the entire point of them.