I'd enjoy it very much, it would be very easy to not get tasks I don't like.
My issue with it that it's that easy. It would make it significantly easier for me to get the task I want. I already get more slayer points than I spend cancelling, if I were to be able to block tasks that would be even worse. I would gain a ton of slayer points and basically wouldn't be able to do much with them aside from buying hcb's. So no, I'm gonna say no. I'd rather keep spending my points, even if it's a whole lot easier to just block.
I'm currently at 245k slayer points and I need something useful to spend them on, blocking tasks would only cause that amount to increase faster, this is why I won't agree with it despite me probably enjoying it.