#1 -- It should be better scorer rather than better offensive player. Bird's passing ability would lift him clearly ahead of Pierce even if Pierce was a marginally superior scorer.
#2 -- I disagree with it but I do I think Pierce has a legitimate case as the Celtics best scorer of all time.
Like Moiso said, they played at a faster pace in the 1980's, roughly 10% more possessions per game. Drop Bird's 28ppg (done three times) by 10% and you get 25 points a night which Pierce has hit 5 times in his career. Bird's career high of 29.9 becomes 26.9 which is comparable to Pierce's career high of 26.8ppg.
Also, Pierce is right in line with Bird's career shooting percentage at 56.5%. Pierce has also been between 58-61% TS% for five of the last six seasons whereas Bird hit that range four times. Furthermore, I fully believe Pierce would have posted better TS% figures earlier in his career if he had played with more offensive talent around him.
Ergo, I don't see a justifiable case for Bird as the more prolific or efficient (by TS%) scorer versus Pierce. I think they are both in the same ball park.
#3 -- That all said, I would take Bird as the best Celtics scorer of All-Time.
His post up game was unstoppable, he was one of the best of all-time off the ball. He caused the defense to collapse at will, more so than Pierce did. I would rate Bird as the tougher cover, the tougher guy to limit/stop. Bird created more of a mismatch problem.
I also think the way he scored (especially off the ball work) was more conducive to his teammates' offensive performances than Pierce's slashing + clearouts game.
Like I said earlier, once you take Bird's passing into account, he is far and away the superior offensive player.