Because that's not how I conceptualize weapon training. I imagine the Fighter or whoever spent some extra time to get BETTER with a weapon, they didn't just spend extra time to not be bad.
Funny, it's
exactly how I conceptualise weapon training. You need to learn the basic techniques to not suck before you can learn to be better.
And for most characters, the weapon proficiencies that they have are the ones they gain for free with their class, so they didn't spend any extra time not to be bad; they spent that time in their basic training for that purpose.
It's a way of framing proficiency that I feel gives value to proficiency, without knocking down a player for wanting to play with the weapon of their choice.
For those players who want to use an offbeat weapon choice, it should be possible for them to spend a feat to get the appropriate proficiency, just as in 3e and 4e. Options, not restrictions.
But that's not the baseline case - most players will naturally stick with the weapons given by their class.
And who cares if the wizard runs around with a greataxe or a longsword, their BAB is what, 2, 3? HALF that of the Fighter? Who if we give a bonus for proficiency is now probably sitting at triple?
See that difference in BAB between the Wizard and the Fighter?
That's the extra time that the Fighter spent to actually get BETTER with his use of weapons. He doesn't need to be getting yet more bonuses on top of bonuses.
Two further reasons why it's a good idea to treat proficient weapon use as the baseline, and so not give out proficiency bonuses:
- It allows the Basic Set to omit the rules for proficiencies altogether. "Wizards can use quarterstaffs, daggers, and slings", or whatever. Sure, it doesn't cut a huge amount of space, but it cuts
some.
- There's a sweet spot in the d20 system where the total result in a check is roughly balanced between the d20 roll and the character's modifier - where 'luck' and 'skill' are balanced. This means that in most cases they should be aiming for characters to have a modifier of between about +5 and +15 in the things that they are good at (because those are the ones they use most often).
If the game gives 1st level characters large bonuses (ability modifiers on top of proficiency modifiers on top of BAB) then it leaves them with nowhere to go as the levels advance. Given how high the ability score modifiers are (an 18 Str is assumed for all Fighters), they should be cautious in stacking on yet more modifiers.
(And yes, I know that the probability of success for d20+5 vs DC 12, d20+15 vs DC 22, and d20+25 vs DC 32 are all exactly the same at 70%. But the sweet spot
feels better.)
Let characters who are bad at something
be bad at that thing.
Now, all that said, there is an alternative I could happily accept: if the bonus for proficiency
replaces the bonus that the character gets for his ability score.