I'd recommend buying Reforged if:
1. You don't have a legal Warcraft 3 copy as your safest option to get one is to buy Reforged or you simply don't mind spending the money just to try it.
2. You're interested in playing multiplayer, be it custom games or melee, or custom made content made for 1.32 + you like the new graphics + you have a good computer that will run Reforged smoothly + you don't mind downloading/using 25+ GBs of disk space on the game.
I'd recommend using 1.32 without buying Reforged if you're interested in playing any of the content mentioned above and you don't mind spending 25+ GBs of disk space on the game, but you already have a legal copy of Warcraft 3 added to your Battle.Net account and you either don't like the new graphics or you have an older computer that won't be able to comfortably run Reforged HD or you don't find the price acceptable.
I'd recommend using Classic versions (i.e. 1.31.1 or below) if:
1. You aren't comfortable using 25+ GBs of disk space on Reforged.
2. You're interested in playing custom content made for older versions of the game, especially custom campaigns which are not available in Reforged.