Ok, somehow I'm beginning to grasp it.
For one point, Japanese female seiyuus seem to be very capable of voicing different kinds of roles including male roles, you guys have listed many examples above.
The main point, imo is that most of anime main characters are young boys or teen boys, so the softer voices of women work just fine ,or even better, for them, and maybe female voices can be more appealing in many aspects.
Well, in the case of Al, it surely was a smart move having a female voicing him in the Japanese version. In the dub version, they had Aaron voicing Al but his voice...died young in the movie because of some major voice crackage which was very funny. (puberty sucks
) And of course Aaron's fans won't have the chance of seeing him again in this new FMA!