Beyond the desire to defeat the enemy in detail, there is the issue of not wasting lives and efforts on chimeras. As noted in the previous link, the wars we will be fighting for the next half century or so will be ideological wars where we will not be able to forcible impose our ideology on the target population1. This puts a limit on how fast things that are worth doing can be done. As I noted earlier, the first battle in Fallujah ended with the Coalition trying out local Iraqi forces. It didn’t work. However, the key point is that there isn’t much pointing in conquering Fallujah until the use of Iraqi forces is possible. The current push in Samarra is another attempt to have the Iraqi forces demonstrate whether they are ready. I expect that the Coalition commanders are watching closely how the Iraqi forces perform in order to judge how soon to move on other strongholds. Because of the push for elections, we’ll likely have to take some chances because the measure of the Iraqi forces is not just the assault, but holding control afterwards. As many others have noted, the smart jihadis probably just put their guns in hiding places when the assault started and are waiting for the Coalition to move on. It is at that point that the Iraqi forces will need to show their mettle.
1 Some will claim that we are, but until there are re-education camps, population relocations and mass round-ups, such claims are ahistorical blather. Ask the American Indians or the former inhabitants of Soviet Central Asia what forcible imposition of ideology looks like.