If you don't make a special melee attack like All Out Attack, Guarded Attack, Charge etc, there is no reason why a character shouldn't be aiming. You always get a +10 then. So in effect, all standard WS attack are at +10.

I now have players who are argueing the point that they are always aiming prior to a standard attack, even without them having to announce it all the time. Since there is no reason not to aim when making a standard attack. So the +10 becomes a given.

Weird! Non-intended!

Right, at first glance, one (mostly players)) can think there's no reason not aiming, because it's an half action giving +10 to the next attack and simple attacks are also halfs action.

So why not aiming all the time ?

Simply because aiming is an action, in fact !

So, if you aim, you can't move, for example.

You can't feint, either !

And, when adversaries are good at parrying or dodging, feinting is quite more usefull than aiming.

I second that. Ive feinted a few times before and it's saved my bum time and time again. Going along with the feint idea, would you be able to feint in melee combat and then use a standard attack with a pistol? RAW there is nothing saying no. Thoughts?

And if it's not enough to convince your players let your npcs not only feint but give some of the enemies counter attack, too.
That way at least some of them will start feinting, too instead of aiming.

I only aim when I'm worried about degrees of success or if it's really important.

I think I frequently use the delay action. Which I might use to aim if I still can't think of anything to do before my next action.

Aim is good until you get to the higher levels, and have Swift Attack or Lightning Attack. It also precludes moving, charging, feinting, grappling, disarming, All-Out Attacking, etc. It's a decent option but after a certain point it's just one decent option among other (often better) ones.