That isn't a great argument puggy... one could easily argue that a smart opponent can prevent such an opportunity from arising in the first place.
Not exactly. There are 6 pokemon on each players team. But the thing is, only once they have been sent out to battle will they be revealed. Seems obvious but unless the opposing player has the ability to see the future (which is rather uncommon), the 6 pokemon are a mystery until they are set out. This means a memento user will be a surprise.
The most broken of these is Jumpluff who has encore.
For example
Dude 1's Yanmega used something
Blissey used Ice Beam
Dude 1's Yanmega fainted
Dude 1 switched in Jumpluff
jumpluff used Encore
Blissey used Ice Beam
Jumpluff survived due to focus sash
Jumpluff used Memento
Blissey's sats were lowered
Jumpluff fainted
Dude 1 switched in Charizard
Charizard used substitute
Blissey used Ice Beam
In the end, Charizard sets its self up and proceeds to sweep the opposing team.
Really, this tactic can be likened to the use of Wobb in todays metagame (remember, no switching). Wobb also has no true counter as we know it, as nothing can switch in to it. Even the best players can be beaten by Wobb, Kira. And really, we all know how popular Wobb is.
Even if this isn't such a great argument, it's better then "Deoxys E outspeeds everything!!!!Therefore Memento shouldn't be a problem!!!!!!"