Im going to repost this from a comment i made in a youtube video, if you see it there, it is mine and dont flame me

To all those that are saying this is cheating, you have absolutely no idea of what your talking about. It is mechanically engraved into the game, let me explain. Every ability has a duration to its animation until it reaches the point where the damage is calculated when it connects, either over the course of its duration for channeled skills, or at the apex like uppercut.

When uppercut for example, hits the end of its hidden cast bar, the damage is calculated and delivered, after the cast ends, there is a recovering animation or "follow through" based on ZOS, ie your character will "sway back" the weight of the 2h and re-balance. During this recovery animation, the player is BY DESIGN given the freedom to take another action, these actions are basic attack, block or dodge roll. This was done so that combat does not feel barred or restricted artificially.
As a player lets say you successfully land an uppercut on a target, but right as the damage connects, you notice an incoming aoe on the floor, or a snipe incoming and you have the ability to roll out of the way or quickly block the ranged attack. Those that claim animation canceling is cheating and should be removed from the game, are advocating that your character should be rooted for the duration of the recovery animation, despite already earning the pay off of the skill by landing it and taking the hit from the incoming attack. This removes all player agency and punishes those that play well and disallows good players with fast reactions or awareness the opportunity to excel.
And as for its actual effectiveness in pve rotations
there is no reason to block cancel in pve whatsoever, unless you need to block an untimely attack in the middle of a rotation. If you hit steel tornado and held light attack, the game is going to queue up and deliver it as soon as the game itself allows thanks to an internal GCD. If you hit block immediately after steel tornado then try to weave a light or medium attack, it will not come out any sooner than if you had just held attack right after casting it, there will still be a small delay. No matter what. Basically, blocking, basic attacks and roll dodge both occupy the same space between abilities, they are off the GCD, and they cannot be used to increase the frequency of which you are allowed to use abilities on your bar.
Block canceling can help with an individuals timing however, espeically when light weaving as sometimes it gets a little tricky depending on the abilities being used. But block canceling can not by execution increase the speed of a rotation involving light and medium weaving beyond what the game allows on its internal GCD.