I agree Peja's performance is dreadful after the Lakers series (I feel that he got his revenge and is now content). That said, I don't question Carlisle's decision this year. Mavs don't stand a chance against the evolved Heat. The Miami team is one difficult team to play against - suffocating defense + two super players who have proven they can step up when needed. That said, Mavs' offense power is unstoppable if catch fire, and the only way to beat the Heat is not to play trench warfare defense, but to use all the firepower they got. In this sense, using Peja is a great gamble, but one that may (or may not) pay off.
In fact, Mavs had the potential to grab G1, but couldn't do it because both Terry and Peja are off. Just look at Brea. He has improved so much I am very impressed with him this year, but he couldn't do a thing against Heat's defense. Brewer will face similar fate in terms of offense. That said, I can't deny the fact that LBJ can somewhat be defended. Remember last year when Celtics sent Tony Allen on LBJ? Although he could not stop LBJ, Tony distracted LBJ enough that he produced one of his worst playoff records (the triple double is what people call "wash record" - good on paper but useless to win the game). My question is whether Brewer is on the same level as Tony Allen, especially given the new LBJ is a complete different beast in comparison to the way he played last year.
Overall, I say using Peja is still a rational decision, even if it is a gamble at best.