Fagert, when u say alpha map, u mean bitmap mask ? In that case it would be very efficient. For example, if you want to make a circular Radar like u can see in lot of games, u just apply a circular shape as a mask. For tiled terrain transitions it would be very fast and very nice looking, you would just have to draw your masks (all the different kinds of transition, for example a rounded shape for transition, or just a linear transition ,...) on photoshop or whatever then you can apply them on the first image (grass for example), then u draw the result on the second texture (stone for example) and you get easyly the expected transition.
If it's badly explained, here is what it looks when it is done tx to photoshop: