The VT-340 didn't support ligatures, therefore, we shouldn't use them ;-)

Now, seriously, a programming font has no (and should never have) awareness of the computer language it's representing. Ligatures make sense if they present meaning in a way easier to read than the non-combined characters. I can't type := in APL and expect the REPL to think it's an attribution arrow (←).

I get APL is a rather extreme language to talk about, but this is one example where ligatures would harm the programmer (unless your editor replaces a ":=" with a "←"). Source code is a very precise language and the choice of characters you use matters a lot. A "<-" doesn't mean "←" unless the language declares so.