And what Flux posted makes the most sense. After level 60 you get exp and certain points you get more MF & GF. What comes to guarantee legendary is bs, Paragon levels is added to compensate missing level grind.
Well I remember a ton of casuals whining because they weren't able to get NV stacks while leveling characters. This might be an answer to those cries giving both bonus experience and magic find. I guess this paves the way for future expansion(s) when the level cap is raised streamlining the entire character leveling experience. That's just a wild guess though.
Some of those "Casuals" as you put it are Hardcore players, who often have to level new characters to replace the ones that died. Being able to get NV stacks would be useful for players below 60.
Also most Hardcore players would make softcore players look casual, nothing makes you understand a game more than having your very survival depend on that knowledge.