Creative instakilling is indeed a very bad idea, but I fail to see how you think this is too. In my opinion, attack speed shouldn't even be a thing to begin with, and combat should remain how it is in 1.8. But I'm not going to argue with the devs about their decision to put attack speed in. What I DO want is for creative mode to have less hinderances, and having attack speed apply to it would do much more harm than good. Like people said earlier, if you want to have attack speed on "simply for testing purposes", then switch to survival. And so what if you have to give yourself a few potion effects? It's better than having to constantly monitor your punches in creative because swing speed does apply.
Also, you compared my idea to creative instakill, when I personally think it would be better to compare it to another mechanic of attacking: for example durability? Saying swingspeed applies to creative isn't much better than saying durability should apply to creative. One could easily argue (as you have about this) that you should be able to test out durability without going to survival, but in my (and many other peoples') opinion, durability not applying to creative is a good thing. Well, I happen to think the same thing about this.
I miss missing you now and then Sometimes before it gets better The darkness gets bigger The person that you’d take a bullet for is behind the trigger Oh We’re fading fast I miss missing you now and then
But still, attack speed helps to determine DPS and the best timing for attacking (given crits, blocking, etc.), just like damage, while durability you can just look up in an instant on the wiki, and it would always be annoying in creative because it applies to pretty much everything with durability, there's no potion effect you can give yourself to solve the problem.
But honestly, if they do remove attack speed then they SHOULD make every sword instakill.
And so what if you have to give yourself a few potion effects? It's better than having to constantly monitor your punches in creative because swing speed does apply.
The point is that you have to give yourself like 10 different potion effects to get creative mode but with real attack speed with this change. And even then, there's no way to simulate flying in survival mode.
Meanwhile, if you want attack speed off in creative and your change is not made then all you have to do is give yourself strength 100000, or maybe an attack speed 1000 sword. Which is pretty much the way it is right now for instakilling, so why not for attack speed?
Yes, it could be an optional gamerule. But that probably causes lag, which minecraft seriously doesn't need more of, and what's more it would have to be per-player, which means 100x lag on some servers.
Also, it's counter-intuitive. If people pull out a non-sharpness X or strength 100 sword then they probably want to test how much DPS it has, not just get rid of mobs quickly.
You seem to think that attack speed means that you kill stuff slower. Not really, the DPS of diamond swords in 1.9 is pretty much the same as the DPS of diamond swords in 1.8, so attack speed isn't making it less convenient as long as you're used to not spam-clicking (which people will be a little after 1.9 comes out)