Actually, at HI it can, because most HIs do not offer free breakfast to everyone, but some (but you have to research each hotel specifically!) do give free breakfast (of one sort or another) to Plats. In fact, because I do research and only choose the hotels that do, I have a 99% success rate at getting free breakfast at HIs. (My 1% failure was that my research did not show that there were day-of-week restrictions at one particular HI, so one of my two days there I didn't get it because it turned out that that HI didn't give free breakfast to Plats on weekend morningss, only weekday mornings, and I had a 2 night stay Sat-Mon.)

It's at HIX, Staybridge, and Candlewood that I'd say Plat status doesn't get you much. (Except HIXs outside the US may not give free breakfast and/or free internet to everyone, so in those cases it's more like I just said for HI, ie, up to the individual hotel.)

Not necessarily, because again in Priority Club it's up to the individual hotel what, if anything, they give to Plats. And if they don't give you free breakfast and/or free internet, the breakfast and internet charges can sometimes be way higher at a CP or especially IC than they would be at an HI down the streeet. So if your total goal is to pay the least possible (nothing if at all possible), then once again you have to research the individual hotel and figure out what they give Plats and what it costs for those things they don't give you as a Plat.

(Now, if you're not going to eat breakfast in the hotel anyway, and you're not going to use the hotel's internet anyway, those issues may not factor in for you. But if so, you need to explain that.)

Upgrades always "depend on availability", so while Priority Club does guarantee you the access to upgrades, it doesn't guarantee you that you will actually get an upgrade. But all too many Priority Club hotels don't even try to upgrade you, while others give great upgrades (though not necessarily on every stay, because again it "depends on availability").