HI, Crossunder: "Positive Rate" - Yes we use it - though some have their own variation "Positive Climb" from their own prior backgrounds, despite what it says in our manuals.
Yes, also seen it go wrong in the sim under pressure when the call "Positive Rate" perhaps gets forgotten.
On the whole, a prime purpose of callouts could be said to be focussing the attention of the caller on what exactly must be watched, so that the call can be made. The rate of climb and some evidence of being higher must be monitored closely before raising the gear.
You are quite right to point out that this call can degenerate unintentionally into a prompt by Pilot Monitoring (PM) for the Pilot Flying (PF) to call for "Gear Up". As you say, "an error-prone trigger, especially in an emergency". Without the prompt, you as PF might discover under stress that you had trained yourself not to bother thinking to call for the gear, just to react to the prompt and respond with a call for the gear. Thence the situation you and I have seen in the sim, of 2 intelligent pilots a minute later amazed that even at full thrust on the one engine, it still will barely climb, not even noticing the three accusingly green lights.
One remembers when we introduced the "silent cockpit" principle during engine starts. There was no more need for PF to make sequential calls like "N1, ignition, fuel flow, EGT, stabilised ". Apparently these calls might distract other crew member(s) who might miss something they would otherwise have seen - good sense. After a while it became obvious that the critical focus on those parameters was now lacking, and they were often not being monitored well or at all, simply because the parameter callout was not required any more.
We found a good answer was to train yourself well, probably by making the parameter calls in your head to yourself at each stage of the start as PF, just not verbalising them so as not to distract the PM(s). That gave us the best of both worlds - the critical focus, and the 2-crew (or 3-crew) undistracted principle of the silent cockpit.
This can work well in the "Positive Rate" trap you describe: you must train yourself as PF to ask yourself "Going UP yet ?" every time you pull back to raise the nose to the climbing attitude. Train yourself that when you have answered "yes" to yourself, you will say "Gear Up". Make sure there is no requirement in your airline for the PM to make the "Positive Rate" call BEFORE you as PF call for "Gear Up" - when you as PF see rate and height gain, call for the Gear. Meanwhile the PM will have been focussed on height and rate waiting to make the "Positive Rate" call, will soon complain if there is danger, won't raise the gear if there is, will raise it if all is well. Your "gear Up" call will have broken any pressure-induced trance PM might momentarily have been in, befuddled by pressure.
On the whole, I vote for the "Positive Rate" callout, because of its value as a focus tool, recognising the danger of it becoming a prompt, and I use it as described above with my silent additions. The lack of a callout requirement would probably mean my failure as PM to focus on an essential, especially under pressure, which would be worse.
Now - did I re-engage LNAV after that radar heading ? Perhaps a call-out would help ?