On a crowded platform they're obviously going to be able to pick them out more quickly. The clue's in the name.
Even on a crowded platform, if I can't easily see the dispatcher, I'm not even thinking of starting the dispatch process. I have held dispatch at Euston for this exact scenario recently.
Until that PTI is 100% clear and I have a clear sight of a dispatcher, I'll be watching the crowd behaviour/clear away.
Don't need hi-vis, if you can't see the dispatcher without hi-vis, shouldn't be taking the tip.
Avanti using LED battons certainly helps in stations where bleaching is a hazard, or in low light situations. Helps more than a hi-vis as the vest type doesn't highlight the arm position unlike a batton.
Also, I see many different roles where orange hi-viz such as drivers, platform staff, station managers, Network Rail staff, Alston/Bombardier staff. If one of them sticks their hand up from 8 coaches away (160m) should I take it? Use of flag/lights/battons will always be the safest way to dispatch.