I remember loving the Arroyo trade when it was made. Pena had big time power, needed a little work... but it ended up being pretty bad. Arroyo was signed to a more than reasonable 3 year deal, and had a solid run over that time.
There's been a lot of talk made about Ben being a gambler, and I tend to like that mentality... this was one of Theo's gambles, and one that didn't really pay off. It seems to me though, that when Theo gambled he did so wisely. Maybe that's just hindsight, but he traded for Jeremy Giambi and signed Ortiz gambling that one would become a legitimate DH. He hedged his bets with two guys. He had Shea Hillenbrand who was pretty good in 2002 (4.2 WAR according to BBRef) to play 3B, but he added Bill Mueller for peanuts. If Mueller played well and Shea played well Shea could DH or move to 1B... Basically he had a lot of moving parts that could play off each other. When Ben gambles it seems like he does so without a decent backup plan.