I think the standby mode is really there to stop the transponder from transmitting while you change codes preventing inadvertant emergency code selections. I still teach my students to switch to standby while changing codes. I know that most ATC SSR heads now have a few seconds delay before displaying - and in the case of emergency codes - actioning indications, but IMHO it's still good practice should you not change sqwawk fast enough or get distracted part way through.
As standby is there, I see no reason not to use it on the ground prior to departure - at least the kit will be warmed up.
Some larger airports are now ditching their ASMI (airfield surface movement indicator) radars in favour of a new system which uses mode S. LHR is one of these, transponders are now set ot the start of taxy and go off when on the gate.
You won't get a TCAS Resolution Advisory on approach from an aircraft holding as RAs are inhibited below 800' on most TCAS systems. On the aircaft I fly, you get no audio either. You just get the yellow traffic squares with alt readout. A Go Around won't happen in this situation.
As an aside, the word 'Squawk' comes from the wartime codes for Identification Friend or Foe (IFF). During WW2, most classified kit was given a codename. In the case of IFF (which is still essentially the same as SSR, just a slightly different frequency and only 76 codes) it was 'Parrott'. Your 'Parrott' replied to an identification request by 'Squawking'.