They are punishing early buyers. I got ripped off during the summer sale, bought the season pass and then I found out that it wasn't a "season pass" and that the GOTY would be the ideal choice for just 1€ more. I only wanted the Tiny Tina's addon, so no big deal.
In short: Logic calls out buying the season pass if you own the base game, but you pay more for less... and you also payed more back in the day.
...and I must agree with some posts here, Borderlands 2 is a overrated borefest.