I'd hardly consider this a castration of the Brigand deck. Prince of thieves is useful with nearly every deck due to its low cost and the fact that its effect is independent of what else you have in your hand. Causing you to wait a turn does little to lower its efficacy, and if the alternatives are to raise its cost (or would lowering it make it worse?) or reduce its maximum/minimum effect, I'd rather just have to wait that turn. It gives you more time to broaden the gap in your relative stock levels.
I doubt anyone will stop using the card because of the change.
On the other hand, it's worth questioning whether this change is truly necessary. Does it break the card? With the aforementioned Dragon squadron, your opponent has one turn to prevent its fatal use, and that can be accomplished by discarding it, raising his defenses adequately (I have had an opponent survive its use before), immediately defeating you via any victory type, or decreasing your recruits. With Prince of Thieves, your opponent now has two turns to discard it, win the match via any victory type, or spend as many of his resources as possible to lessen (but never actually negate) its effect. Assuming the latter option, you are granted that same opportunity to deplete your stock, so the only true advantage is one additional card draw that might allow your opponent to force its discard.
It is also worth noting that it is currently impossible to draw Prince of Thieves, a rare card, after a Quick or Swift card, so the opportunity to draw it and play it before your opponent could react wouldn't exist even without this change.
Really, the only real effect I think the change is having is that it is catching people off guard and causing them to waste its use, resulting in understandable frustration. If the "No effect if New" addition is going to stay, I feel it should be the first line of text on the card rather than being hidden away at the bottom.