Ligatures in programming fonts | Butterick’s Practical Typography
I can see (pun intended) Matthew Butterick’s point. Perhaps a better solution would be to have better kerning—the horizontal spacing between characters on a line—for those combinations where you want them closer together to show the connection.
On a broader note, for me ligatures in typefaces can be a mixed blessing, as you’re dependent of the skill of the type designer. I’ve seen some examples where they can create nice effects, for instance having one letter duck under another, but most of the time you’d want to pick and choose where to use them. On the other hand, some ligatures can stick out like a sore thumb. (Matthew mentions the Th ligature in a lot of Adobe fonts.)
What’s really annoying for me, though, is that ligatures in text in PDF documents can cause havoc when you copy the text and paste it into another application. It becomes pot luck as to what the ligatured characters turn into. That has caught me out more than a new times in the past, and I now know to double-check afterwards.