Let's get to the basis. Coffee inhibits Adenosine. Adenosine is a brain chemical that accumulates itself during daytime to make you drowsy and put you to sleep at night. You are right when you say using coffee during the day while taking Adderall or Dexedrine may seem useless, ok. What if you're skipping a night or two in a row? Do you have any idea how much adenosine you'll have in your system? What I personally do is I combine 100mg of caffeine for each 10mg of Dexedrine, because, from experience, it is a high enough amount of caffeine to keep adenosine from causing sudden drowsiness and even narcolepsy upon amphetamine crashes, and low enough to still benefit from adenosine's neuroprotective effect while binging on amphetamines.
So it's a good combo if you know what you're doing, terrible combo if you don't.