@zidroc
I played on 2 servers back in the day, simply because I had too.
I played on PvP server with my first main (undead mage). I played in one of the better PvP guilds. Organized matches with teamspeak and people knowing not only how to play their own class, but also how to play as a team, was super fun.
However, most of people on PvP servers were focused on, yeah you guess, PvP so there weren't any super successful PvE raiding guilds there. PvE raiding there was mediocre at best. So I had to have another main (nightelf druid) on a PvE server, enlisted in a raiding guild that raided the high end content.
That is how I was able to have the best of both worlds, raiding with PvE guild when raiding days were, and rest of the days, having real PvP matches with a PvP guild. (PvP matches on PvE server happened, but they were mostly PuGs, and organized teams there, honestly, just weren't on the same level as teams on PvP server).
So having characters on both (PvP and PvE) servers was a must back then, if you wanted to do both, top PvE, and have an actual challenge in PvP.
Now days, at least on private vanilla servers, people are all experienced veterans and know how to play both PvP and PvE, so you can have it all by playing just on a PvP server, both raiding and PvP.