But this is where it comes down to the composition and fit to me.
If we're working from a hypothetical standpoint that the team is, for all intents and purposes, healthy, then it comes down to how those pieces come together. Technically you can say players x and y can account for z's productivity but it's how that productivity translates to the court and their teammates. So when you compare the Celtics at full-bore and the Spurs at the same, even if you could make an argument for the individual productivity comparison, it doesn't translate the same; the Celtics have a pass-first point guard whose M.O. is to get in the paint and he's complimented by plenty of shooting with three perimeter-oriented stars and his two Bigs might as well be lead blockers. Players get the ball in their spots at the right time and the sum of the group is greater than the individual because each of their jobs compliments and enhances the other on both ends of the court.
Now if you look a the Spurs' Top-5 (assuming Splitter's in it), you've got a slashing, score-first point; a slashing, playmaker at the 2-guard; a slashing, open-court player at the three; and their front court consists of two pretty conventional post players. But I'd actually contend that Hill is in their Top-5, so that'd give them one spot-up shooter (which would help the cause offensively) but most often leave them at a defensive disadvantage (which they'll often have whether it's Hill or Jefferson). You could say Duncan and McDyess could very well both be perimeter-oriented Bigs this year (that is what 'Dyess is and there's always the talk of easing Tim's burden by keeping him at the high post more often) but the spacing is still not all that conducive to the slashing nature of their perimeter (and I'll believe Duncan playing more like Garnett when I see it -- Tim is what he is and talk of him playing almost exclusively as a high post 4/5 is just that. Talk).
So whereas the Celtics have the size and positional balance on both ends of the court from their Top-5, the Spurs have some fundamental flaws. It's not a fatal flaw if they can find a couple of players to put the pieces in the right place and find that positional balance, but it could be if they can't manage to do so.