For some people it's to learn the new mods that have 1.5 versions that will be in 1.6. There are going to be a lot of changes even from 1.5-1.6, much less 1.4.7-1.6. Just look at all the newer stuff for MPS. Add in the new mods such as Natura, Tinker's Construct, MFR's Rednet, Extra utilities, as well as any other mods that may be added and you could see why some people would want to familiarize themselves with what's available. Let us not forget either than 1.6 most likely won't be compatible with 1.4.7 worlds due to changes made in some mods such as TE, as well as new world gen.
Personally, I would like to see Ars Magica in but I can understand why they're against it due to it having similar systems to the old EE2, but if they are able to modify the configs in a meaningful way to make it so, say, making 400 Igneous Extruders is not viable for a diamond farm using the conversion factors then they might be able to put it in (though this would probably take a lot of effort to balance properly hence the reason they don't want to put it in). Another possible addition would be UE with Mekanism, though these might be seen as just adding extra bagging since it's basically another power system and more thing to work with it a la buildcraft or industrialcraft. The bonus with UE is of course the built in method of power conversion.
Basically, we don't know what all will be in or out, what all will be changed, and we know that 1.4.7 will most likely have a hard time being upgraded to the 1.6 version of the pack, so some of us want to mess around with 1.5.2 to get a feel for things, especially since a 1.6 pack will definitely be a long ways off. Watch Mojang release a stable version of 1.6 as FTB gets a stable version of 1.5.2 -_-
