You still get somekind of priority if you buy Y,
- PL with Y will be upgraded before PL with S
- GL with Y will be upgraded before GL with S

As for those who use to buy the ticket same day of flight, chances
for upgrades are lower than if you buy it 72 hours prior to flight (as they
might upgrade people in this 72 hours...)

So the winning formula is buy a Y Class ticket at least 72 hours prior to the flight...

BTW, For those of you that work close with an agent, if you book a Y class
at least 7 days in advance there is a promotion Y class price, which might
be even cheaper than S...., Used it already twice.