I don't see what you're getting at. Sure Mewtwo was able to abuse it, but that was likely intended as it was printed alongside DCE. Since then DCE has been appropriate with most cards. Keldeo can use, but doesn't get the full effect of its attack, Lugia can use it, but still needs Plasma energy, and a number of other cards CAN use it, but it isn't worth it. It isn't really an abusive card right now, and unless the direction changes to a DCE heavy format (like HGSS) I don't see that changing significantly.
In spite of not understanding it, you come pretty close to grasping it.
It is turn 1 Energy acceleration, which I believe is very bad for the game. If you hadn't noticed elsewhere, I don't care for the new first turn rule because it is throwing the baby out with the bathwater (to use that old expression): attacking first turn isn't the problem it is what attacks are available. Factor in how we were told that the current first turn rules (essentially the original first turn rules) were instated because it was too "complicated" having different rules and how "set-up" attacks actually improve the game's pacing (slower than instant offense, faster than depending only on Trainers) and hopefully you can understand that position.
So for
Double Colorless Energy to be "safe", no card should be able to use it to attack first turn, which should be accomplished by pricing the attacks correctly and not erratum or rules modification. Further, all cards need to be designed with it in mind regardless - it is simply quite potent and as we saw, dictates much of the game. Is it an immediate problem right now? No. Has it been a problem? Yes, as you stated. Have you considered why it isn't much of an issue now? It simply isn't useful to enough of the more potent cards in the format. You brought up
Mewtwo EX yourself; what would that card have been without
Double Colorless Energy?
If something is prone to causing problems, it is best to eliminate it from the game, at least until stability has been achieved and there is room for such experimentation.
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Tropical Beach can't be in the set.
That would make it legal for 2 more years.
Explain to me why this is a problem.
My criticism of it is merely its rarity; the effect is reasonably well balanced, and even the rule changes will not make it an issue. Though I once opposed "staples" vehemently, my stance has softened as sometimes there are only so many ways to execute a mechanic well, and it seems silly to waste time reinventing the wheel just so each deck can do it in its own way.