This has been a good discussion, although I'm still not entirely convinced, as my big worry isn't T3/T4, but T5/T6. To me, the worst-case scenario of A/A just seems a heck of a lot worst than the worst-case scenario of A/S/A:
A/A Player T3: Draws A/A/C/C/C -> returns two coppers, gives opponent 1 copper. No buy.
A/S/A Player T3: Draws A/C/C/C/E -> returns two coppers, gives opponent 1 copper. No buy.
A/A Player T4: Draws E/E/E/C/C -> No buy.
A/S/A Player T4: Draws S/C/C/E/E -> Buy Ambassador.
A/A Player T5: Draws A/A/E/C/C -> returns two coppers, gives opponent 1 copper. No buy.
A/S/A Player T5: Draws A/A/C/C/E -> returns two coppers, gives opponent 1 copper. No buy.
A/A Deck: 2x Ambassador, 5x Copper, 3x Estate
A/S/A Deck: 2x Ambassador, 5x Copper, 1x Silver, 3x Estate
As you can see, worst-case the A/A player hasn't bought a single card between turns 3-5, while the A/S/A player was able to pick up their second Ambassador and is a Silver ahead. Sure, the odds of this worst-case scenario aren't high, but I really don't see the A/A player catching up at this point.. they're a full turn behind the A/S/A player by T6. The A/A player *may* outpace the A/S/A player with better luck on collisions, but is it really worth the risk of possibly capitulating at T5?