Biomes O Plenty is included in the Direwolf20 1.5.2 modpack "to provide the option to use," but it is disabled by default, presumably because it makes drastic changes to world gen. However, I think that it should not be disabled by default, for several reasons.
Firstly, when creating a new world, BoP gives you a choice between standard and BoP world gen. The default is standard terrain. Additionally, the Direwolf20 1.5 pack is based on "Direwolf20's experiences on the Forgecraft server." BoP is enabled on Forgecraft. Finally, for those (like myself) who play with BoP enabled, it's a bit of a nuisance having to re-enable it every time the pack updates. If you launch the pack and try to open a world, but forget to enable BoP first, you can accidentally mess up your world.
Firstly, when creating a new world, BoP gives you a choice between standard and BoP world gen. The default is standard terrain. Additionally, the Direwolf20 1.5 pack is based on "Direwolf20's experiences on the Forgecraft server." BoP is enabled on Forgecraft. Finally, for those (like myself) who play with BoP enabled, it's a bit of a nuisance having to re-enable it every time the pack updates. If you launch the pack and try to open a world, but forget to enable BoP first, you can accidentally mess up your world.