Perhaps I expressed myself poorly. What I meant that if you choose to take, say, Rafiel into the tower, and leave Rayson and Leanne outside, then Tibarn and Naesala would be left outside as well. As in making it so that you can't have a team full of laguz royals. I wasn't really thinking about it as forcing you to take one of the royals with you, but I guess it would have that effect as well, so I suppose that is a good point.
Speaking of the herons, though, wouldn't it be nice if it was actually possible to get some of the higher level galdr in normal gameplay without dumping a ton of exp on the them? Either have the herons start out at a higher level, gain experience at a faster rate or have them learn the galdrs at lower levels. Now that I think about it, it would also be nice if each of the herons could learn their own unique galdr as well, kind of like how spells work in SoV.