When to acknowledge ATC

A Guy Called Tyketto wrote:

If you're working in any sort of Class C or D airspace, his
response to you is in fact clearance into that airspace.

No clearance is required.

So if he
responds, you can fly through unless ATC tells you 'remain outside
Class x Airspace'. Then you must read back something.

The last sentence makes no sense whatsoever. If I call and he answers,
that's two-way communciation and that's all that's required.

Class B airspace
requires ATC to tell you that you are cleared into it. So if ATC tells
you 'radar contact location, altimeter xx.xx', you're cleared through
that space where he's controlling, establishes 2-way comms, in which
you need to acknowledge that he hears you whether it's a vector he's
given you, or just replying with your callsign, that is all that's
needed. You can reply back with the alt. setting, but it isn't really
necessary.

RADAR CONTACT is not required. Altimeter setting is not required.
Readback is not required. You know he hears you because he responded
to your radio call with your call sign. I'm having a hard time
following your argument.