its for when you are holding the key, when you press another, the overlap. useful for scouts so they can move quicker but for me it fucks up rocket jumps.

Sometimes you accidentally cancel the momentum of your doublejump if you don't have null cancelling.

So is this mainly a scout thing or is there some use in this as a medic main apart from just preferring it?

Arguably less wrist strain if you enjoy wiggling around like a slippery snake

Don't really think it can help or inhibit for any specific classes. I think it's more a personal preference thing, though I can agree that it's effects are felt most on scout. Still, I know plenty of scouts mains with godly DM that don't use the script.

Iirc most invite players don't use it for the record. Some do, but it's all down to preference/what you're used to. If you learned good movement without it you probably won't see any benefit from it.

A lot of the ideas for scout apply to medic (you don't move as fast but you still strafe, arguably you need to switch directions faster even) except with surfing instead of doublejumping.

http://www.teamfortress.tv/36460/a-few-questions-about-null-movement-scripts

A lot about this topic was discussed here, but if you don't want to go through and read all of the posts made, basically it says that there are a number of invite players that don't use it mainly because they've gotten used to playing without it. It will take some getting used to if you're looking to pick this up as your preferred method of movement, but it's not impossible to play without.

Personally I like it when playing medic because I feel like I have to put less work into making my movement unpredictable, but honestly I think I could do without it when playing other classes like demo/soldier/scout. I just use it mainly because I've gotten used to it over the few thousands of hours I've put into the game

Null can be a hindrance sometimes because if you're strafing against a scout, he'll expect you to move back and forth, jump over stuff, etc. However, sometimes if you press A and D, you can stop and go, which can rush shots and is a popular strafing method.

I feel like it's harder to play scout with it. Strafe aiming is really weird with it because if you tap the opposite strafe key you suddenly jolt the other way and it can be off putting for my aim at least. Without it you can get the timing down and be as responsive as possible but if you want to slow down strafing left for example you can just tap a instead of having to let go of all strafe keys which is what you would have to do with the script. I also find that sometimes I will accidentally strafe cancel with the script when I'm in the air and jolt myself to one side or the other as scout. I feel like it's a lot more fluid and less jolty without the script and the only real benefit is being able to AD mash and make your model look fucky (which is sometimes useful especially on medic). I feel like it and a couch jump script make it much easier when you're newer but can be detrimental or less useful than taking the time to learn the timing of things (for example you can't ctap with a crouch jump script and you get knocked back further)

I've been using the script pretty much since I started plying TF2 and just now tried playing without it for the first time because of this thread.
I didn't really notice any difference and never stopped moving by accident.
Probably because I already have the muscle memory to immediately let go of A when I press D from playing CSGO for over 1000 hours.
I didn't see any benefit from not using it either, since when I want to stop strafing for a split second (which yes, can be a good method to dodge against scouts), I just let go of both A and D rather than pressing both.
I don't think being able to stop by pressing A and D at the same time has any advantage over letting go of both, so I'll keep using the script just because why not.

There’s no reason not to have it.
It won’t make or break your game it’s just convenient.