BLASPHEMY! For 25 years, we've always had three hearts at the beginning in every single Zelda game that uses hearts, if I remember correctly, even in the CDi titles (I don't want to count Four Swords here because of the very different nature of these games)!
Nah, seriously, I don't see what's the point in doing that. This somehow downgrades additional heart containers because when you get the first heart container, this means now you're getting only 1/6 more HP while with 3 heats you got 1/3 more. This lowers the motiviation for getting more heart containers and the fact that we ave to collect 4 again instead of 5 doesn't help much here. And of course it makes three heart challenges impossible...
Sure, the enemies may be more challenging and take away more hearts but this is just senseless inflation. IMO they should've stayed at 3 hearts and if necessary cut down the damage per enemy because it doesn't make much difference if an enemy takes out 3 hearts when you have 6 or 1.5 hearts if you have 3. Sure, the 6/3 system gives a higher "resolution" but then again, why not introduce energy tanks like in Metroid with a resolution of 100 right away?