What you place into your various skill slots will vary from player to player and build to build. Depending on your skills you can have both left and right mouse button bound to skills (and pressing 'x' will swap your right-mouse with yet another skill button for a total of 3 mouse binds) and if you don’t have the resources to use the skill, then you’ll normal attack instead until you have the necessary resources to use the skill again. There should be space for, at least, a potion. Diablo III isn't designed with potion spamming in mind anyway, so really, you should have plenty of space on your bars for the elements that will be important to your hero's success.
That still only leaves you with 8 slots. It's a given that 6 of those should be skills. If the player equips a skill, the UI shouldn't be any restriction on actually using that skill. Otherwise it kind of defeats the purpose of having 6 skills.
That still leaves only 2 slots for potions, basic attacks, and scrolls. Basic attacks, and especially potions, should ALWAYS be availible to the player. A player shouldn't have to choose between having a basic attack and a potion. That's just... ridiculous... to put it lightly. And then scrolls are left out in the dust. Or you could have scrolls and potions, but can't benefit from and of the skills/passives utilizing basic attacks. Or worse, the player could be confronted with the idea of only using 5 skills.
If I were the UI designer, I'd simply give potions and scrolls their own place on the UI.
A control scheme that I've found works well, and could easily be adapted to this game, would be to have the skills on numbers 1-6, and potions/scrolls on R and T. This keeps them quickly accessible without moving the fingers away from the main controls.