When using T post and a wire or two gets stretched is is seldom only loose in one spot, it just sags more at one spot. I release the wire on a couple of post, at least one on each side and draw the wire tight then anchor the wire again. If I have to do all the wires there it's not a problem and when I'm done it's fixed.Fine for a single wire and IF the wire needs to be stretched that amount. But with T post fencing often times when you tighten one wire it loosens the one above/below so you end up having to go back to each wire and tighten only enough to make them all taut. That is what makes the 'crimp' method so useful.
I can see that tool being useful in some situations....but in order for it to 'lock' you are committed to stretching the wire a certain amount (which might not be that easy to do or necessary). Crimping doesn't require you buy any special locks/keepers and works well enough IMO. A cow can/will stretch ANY strand of wire that hasn't reached its limits of elasticity.