pierce points:
game 1 24 pts L
game 2 10 pts W
game 3 15 pts L
game 4 19 pts W
game 5 27 pts W
as you can see bro. other than game 2, pierce has been scoring although not that much but it is still decent when your team have many scoring options compare to the lakers.
regarding artest an upgrade from ariza:
ariza although small built but he can score, can play defense, a pesky defender, can shoot compare to artest. like what i have said, the lakers acquired artest because they think that the cavs will be the ECC but they were wrong instead artest has to guard quite a good and intelligent player by the name of pierce. he don't have the body, strength and leaping ability of lebron but he can certainly score and kill you with his step back jumper.
bad day on defense? you don't need terrific skills to play defense, all you need is presence of mind. like what the celtics did to kobe, they know when kobe will shoot although most of his shots are getting in but its all contested. nothing more you can ask.
ariza was a big key during their 2009 championship run and they let him go just to have a body that can withstand lebron.