If you don't see a problem with it then you're already too far gone mate.
It's not necessarily what the one certificate says (although any repurcussions from having/not having one are tantamount to discrimination), it's the potential for abuse it carries.
The government have seen exactly what they can get away with in terms of directly impacting your civil liberties, and if you think a government will simply give up that power then you're delusional.
Today you're getting asked for a vaccination certificate, tomorrow you're getting asked for your papers.
I suppose you're the kind of person that is ok with people's emails being accessed, conversations being recorded etc. on the basis that 'nothing to worry about if you've nothing to hide?'
I was going to compose a through reply to this but then I thought I'd just settle with...
What a load of utter bollocks.