If a player has held his action so as to get two consecutive turns, any NPC attacking that player will have had two effectively consecutive turns to attack that player; also, by saying, “I’ll hold my action”, the GM may then choose to shift the focus of an additional NPC to that player. The GM can even have an NPC hold their action, to counter the PC! It’s not really that unbalanced if it’s GMed right, and the GM can abuse it even worse right back, if he wants to make a point. I’m not convinced that it’s more unbalanced. To me, they are different for an altogether different reason; one is built around allowing for the concept of a counter, and the other is built around genuinely spending time doing nothing. I am generally in favor of allowing a held action that results in two effectively consecutive turns; sometimes, a player wants to do something complicated that he can’t do if he’s interrupted; it’s not unreasonable for him to be able to find an opportunity to. If holding your action costs a minor action, I think that balances things out just fine.
That being said, if a player wants to do two consecutive things in Traveller, he can just take those two -2 penalties for two simultaneous actions; waiting for an opportunity to do that where you won’t be further disadvantaged might be better done as “suspending your turn”. But this doesn’t really model attempting to counter anything.
I think holding an action and suspending your turn until later are really two different techniques for different purposes, and that, while holding an action so that you get two different actions consecutively isn’t inherently broken, it also steps on the toes of the simultaneous actions rule. Holding an action arguably fits better into the exclusive role of countering, while suspending your turn fits better into waiting for appropriate conditions to be met, like a bad guy popping out from cover.