Now the question is: how do you get rid of the KO marker? If it's just by evolving, then a good way to counter it is to keep a good number of evolution cards in your hand ready, or use (super) Scoop ups.
* Imprison & Shock-wave Markers are special markers that were introduced in EX: Dragon Frontiers and will be included in theme decks for that expansion instead of the regular Burned and Poisoned Special Condition markers. After one these markers has been attached to a Pokémon, they are removed ONLY when the Pokémon evolves or devolves. Pre-existing cards such as Double Full Heal or Heal Energy do not remove these new markers an effect would specifically have to mention Imprison or Shock-wave markers for that effect to remove these markers. (EX:Dragon Frontiers FAQ; Oct 26, 2006 PUI Rules Team)
And I theres Tropius from DF to remove them and Holon WP energy would also prevent its effect.
I also strongly disagree with your comment on it being played for late game only. The card does enough damage to KO the majority of basics and stage 1s using Hyper Claws and the ability to snipe the bench before your opponent sets up is very pressurizing.
If you wait for this to be played late game, then your probably going to lose. Why would you want to do that anyway? I doubt cards are made- considering whether they should be played as late as possible. Please explain how 'obvious' it is.
By playing the card deliberately later on it gives your opponent sufficient time to set up and to build something that could be a potential threat against TTar. Also what to stall with? To run a large TTar line and not even play it until your opponent is beating you is just absurd. Every player wants to set up as fast as possible to KO as much stuff as possible to take control of the game by reducing your opponents pokemon, trainers, energy. An early TTar d ex can easily do this by being able to smash anything with a low HP or KO any potential threats on the bench.
The most important factors for building a competitive deck is Consistency, Speed and Power. By playing a strong evo line and deliberately playing it late will often result in your loss.
P.S - Registeel ex isn't really such a good example. Its HP is still quite low even for an ex and with Metal Energy. Wide Laser is very dodgy attack - it can only do 20 and does not have the same back up options that Spinning Tail Tyranitar did (such as additional attacks/evo lines/ATM Rock). If you did get this set up in T5, chances are your opponent will have something bigger active that can do more than 20 damage to you back. Being able to KO your opponents bench isn't handy when theres nothing useful on there and a threat is active. Also the energy cost is very unpredictable to get on T1 - (Rainbow, [M] or [DM] are the only viable choices, once it gets KOd then they will be discarded). Holon research tower only works on d Pokemon. More importantly Aggron ex isn't a good card either - but I can't be bothered to explain right now. :/
P.P.S - Gardy ex never needed to be solely a late game attacker. As well as counting your opponents energy, it also made use of Gardevoir RS's power, Double Rainbow Energy on the bench and Boost energy. As soon as this card was set up (even in early game) it could do massive damage if played correctly. It wasn't a 'game wrecker' like Sneasel/Slowking/ERs and required a LOT of experience and ability to play well so did not deserve to be banned.