Fruitful tension

The last four verses of Psalm 51 introduce a striking - less charitably, odd - tension into the composition. Here they are, split into two pairs of verses:

For you will not delight in sacrifice, or I would give it;
you will not be pleased with a burnt offering.
The sacrifices of God are a broken spirit;
a broken and contrite heart, O God, you will not despise. 

Do good to Zion in your good pleasure;
build up the walls of Jerusalem;
then will you delight in right sacrifices,
in burnt offerings and whole burnt offerings;
then bulls will be offered on your altar.

The first pair broadly represents the interests of the rest of the psalm, but the concluding pair introduces a whole different, and at first glance contradictory, concerns.  To list a few, the psalm has been very individual and personal, but the end introduces corporate and national themes; the psalm is a plea for mercy, but the end introduces the idea that God might be pleased with Zion; and most strikingly - because of the verbal contradiction - the psalm has been concerned with inward attitudes, but the end introduces ceremonial actions.

What do with this?

Of course the standard critical response will be that the last two verses are a later addition, perhaps from someone concerned by the apparent slight on the temple system in the previous end of the psalm.  Well, that's as may be - it's a plausible hypothesis.  The frustration, though, is that by not dealing with the psalm as we have it - and in the only form which we know for sure it has ever had - we can miss out on really helpful theological reflection.  After all, might it not be useful to ponder the relationship between the individual's piety and the good of the covenant community?  Might not a meditation on the relationship between our plea for forgiveness and God's good pleasure be a fruitful one?  And we surely would benefit from thinking carefully about the contrast between sacrifices in which God does not delight and those in which he delights indeed.

I would suggest that the points of tension in this psalm are actually the points where we can learn most about God and ourselves.  I wonder if that might be the case elsewhere?