The Clippers aren't competing now, they're competing in a few years. Pierce is on his way down, and Gay is on his way up.
Well, except we've heard this about Rudy Gay before. He's not really on the rise, he's returned to form, and for who knows how long (Contract Year!). Pierce will still be the better player over the next two seasons. (He's the more efficient scorer, far better from behind the arc and at the line, and a better ball-handler and passer. None of which will change much.)
Instead you're going to have to immediately pay first banana money to a second banana. Ask the 76ers how that can hurt the rebuilding effort.
Pierce is on his way down. He may be better for the next two years - which is debatable in itself - but I'm not competing in the next two years.
Rudy Gay is young and supremely athletic. While there are a few issues with him, there's always the chance that he figures it out. Melo has, and Pierce did before him. It is somewhat of a risk, but you're looking at a draft where three of the first four picks have been enormously risky anyway.
I don't really mind paying first banana money to a second banana if my first banana is on his rookie contract.