What I think: So you don't like the ability to select 16 units. That's your opinion. Glest is meant to be versatile, and not just to modders, but to players. Create an INI option, and for all I care, make it default to 16, but, in truth, we can select any number of units we want. Everyone wins. Those who think they can do better with smaller selection groups are welcome to do so, while those who prefer the ability to select as many units as they want can also do so. By making it their choice, we're standing up for the freedom Glest was created in. The way I see it, tech specific limits things to the perspective of the modder, and that simply won't do.
And of course, in order to be able to select "unlimited" (probably not true unlimited, but have a cap so high that it would be unreachable in Glest), stacking of icons would be necessary.