You don't need Steam to play PG tournaments as the servers are on PP side. You can even install PG and then uninstall steam, that also should be no problem at all. PG installs in it's own directory so there should be an .exe file just like now and you can start playing the game using this exe. Once the game starts you can use the PP Servers and play. No need to have any steam servers etc.
I have found that when I download games from Steam (have over 70) they are put in folders within the Steam directory on my HD (as i made reference to in an earlier post). If you uninstall Steam, then there is every likelihood the game folders within the Steam directory will also be gone.
Steam is just an online portal for obtaining games. You load the games up from within Steam and it runs in the background, but there are no restrictions as to where games are played, so you can easily join a hosted server from within the games own multiplayer lobbies.
I used to play the driving game RACE'07, which I had purchased on Steam. The league we ran was organised by RaceDepartment.com who had their own servers. You fired the game up from within Steam, but once in the game, Steam just sat in the background and I connected to the RaceDepartment servers from the Multiplayer menu in the game.
Many games on Steam these days have achievements, so whilst you were connected through Steam to play the games, as you played the achievements would be unlocked for that game in your Steam account. I see Steam as a PC version of XBOX LIVE or PS+. It's an online gaming portal that allows you to purchase/download games, download free to play games and join Early Access games (which are effectively pay-to-join ALPHA/BETA), but this also means the price you pay to join the Early Access gives you the final full release at no further cost. The other option is waiting until the game releases and paying full price.