And personally, I do mind, which is why I won't use them.
No doubt, you can get money back. And no doubt, it can sometimes be a pain in the butt if things don't go as planned.
And, no doubt, this is only one of many ways you can lose privacy, in a world increasingly marked by a lack of privacy. So it's very hard to avoid giving away privacy and probably impossible to avoid it altogether.
So, I just do everything I can to avoid making it easy for data vampires, and that includes foregoing "cashback", be it Quidco, or Tesco reward cards (or similar), or even using a credit/debit card at all, unless it suits ME to do so. Which it usually doesn't.
We all have a choice .... cash or "free" money. Except, of course, it isn't free. The cost is loss of privacy. And I value keeping as much of my data (like spending patterns) as possible out of the hands of corporate databases more than I value any cashback from Quidco, Tesco, etc.
And while I'm not alone in that, I bet I'm in a minority, and probably, a smallish one.
To the OP .... join? Well, you have, so I'm late to the party. But the answer, for my part, is that it depends how, and if, you value your privacy. I do value mine, so my answer is "no".
YMMV.