@Fortis. I assume that you are not arguing to add Hornets to the Marine lists, or Crusader Land Raiders to the IG - even though there are many historical examples of captured equipment and soldiers being used by their captors or allies.
If I understand you correctly, you would like to have a singular all-inclusive list for each race. How would you reconcile this list with the 'fluff' that presents different themes for the various armies, clans and even planets within each 'Race'?
For what it's worth there *have* been attempts to add 'new' units produced by GW into existing lists - check out the discussions on the Land Raider variants and Dreadnoughts, indeed, the Howling Banshee, Hornet and Lynx debates are still ongoing. Finding ways of making the Tau and Tyranid lists work in E:A have literally gone on for years, likewise the AMTL and the even larger titans, or the pre-Heresy weapons and units.
As the others say, balancing a unit / formation within a Race is quite difficult, often because of the mechanics of the game, cost comparisons with existing units and general questions of 'look and feel'.
Not only that but as Markonz says, GW business practices (to sell 40K models) do not provide a consistent basis for creating army lists. This has been a constant problem within all the Epic rulesets, but especially E:A. Trying to do this would mean that Epic would end up with different 'power' lists like those in 40K, and ultimately in major disappointment to the fans of Epic.
With the greatest respect, we have repeatedly suggested that you *can* add variant units and formations to your battles, including 'captured' Falcons in a Marine list if you feel like it. However, the current list structure is actually one of the strengths of E:A because it gives a (relatively) solid basis to evaluate new units and lists against, as evidenced by the various 'house' lists that also exist on these boards.
@Draccan, I am sure many sympathise with this issue of 'collating lists'. The
Army Compendium is intended to be the source you require, along with
the stats by jimmyzimms. Please post your proposals in the relevant
Chaos thread at the bottom of the TacComs index page.