Another factor in map imbalances is that ice planets are often very weakly guarded. As they have three stages of infrastructure, shouldn't they have the same posible militia groups as desert planets, rather than the less populous volcanic planets? I suspect that this has been requested before, but it wasn't yet on this list.
While ice do have a higher upgrade level potential than volcanics, and crystal is usually valued slightly higher than metal (though that always is a case of supply and demand), classifying it with volcanics is still a good policy. Both volcanics and ice planets require a tech 2 civic upgrade to access, which means it's a serious sacrifice to get them in the early game. This in many ways can make them more difficult and expensive to nab than a moderately defended desert or terran.
Now, the problem is more the massive variability in militia. An ice can have as few as 2 cobalts, 1 flak, and 1 krosov, to as many as (I think) 5 cobalts, 3 flaks, 3 LRM's, and 3 Krosovs. Deserts and Terrans have even more substantial variations. The variations in extractors (2-4) also have a big effect.
Militia could be a topic on its own
Indeed it should. I think there's a lot to discuss before we could put anything up to a structured vote.
Why should the pirates have the only organised defence structure...?
Actually, using the in-game map designer you can actually make a chance that militia will spawn in with tactical structures.