Orc Labs has an RPG game aid application that they’ve got running up on Kickstarter. It’s called RPG Attack.
From the campaign:
Everyone who plays tabletop RPG’s has been in this situation. The game is chugging along as your party goes deeper into the dungeon when suddenly combat breaks out! It’s a fast paced, edge of your seat, life or death situation! Swords are clanging, magic spells flying, explosions, more monsters appear, you are immersed in the experience…
Oh wait, wait, wait, “Hey Bob, did you add +2 to your last attack roll from the bard song?” No? Well that last attack would have killed the monster and then then it wouldn’t have interrupted the Wizard casting fireball and the party would have survived!”
It all comes to a screeching halt. The immersion is broken.
Perhaps that’s an extreme example but we all know that combat often plays out with the players acting like accountants (no offense to accountants). You add up bonuses on fingers, scribble temporary modifiers down on scrap paper, every time you roll a d20 to attack you go through that mental checklist to make sure you’re not forgetting something. Another effect of this is the time it takes to do all that every round of combat.
Think of how much time all the players spend during a session making sure they have done the math properly on their rolls. That is valuable time everyone could spend playing rather than adding/subtracting.