Speed is an asset when the enemy uses primarily hit and run tactics. Since it is impossible to have air support in the vicinity of all our ground patrols in a country the size of Iraq, you need an aircraft that can respond quickly to a close air support request. That is one reason why A-10's haven't been operating in Iraq for the last couple years.
"For the non-conventional combat we are facing in the war on terror it is also nice to have a jet where the pilot can go slow enough to spot his own targets and destroy them on his own without FAC assistance."
That is not allowed, and for good reason. Because of the nature of the combat we are facing in the war on terror, pilots cannot expend ordinance without either working directly with a FAC or gaining clearance through Army or Marine ground commanders. It doesn't matter what aircraft is involved. Furthermore, with the new targeting systems now used on almost every aircraft including the A-10, it is often easier to spot targets from medium altitude with sensors than from low altitude (or any altitude) with your eyeballs. That is why all aircraft now carry those sensors.
"Throw in the fact that the A-10 can survive ground fire much better than other tactical fighters and you have your answer."
That is true, but as with the M-1 tank, the key to its success is to hit the enemy before the enemy can hit it. The M-1's leading advantage against most other MBT's is its ability to fire accurately from long ranges, before the enemy can even fire a shot. There is no need to drive it into the middle of an engagement, slow it down and have it absorb multiple rounds if it can destroy the enemy just as effectively at range. The same is true with fighter aircraft. Our most effective (ie. accurate and destructive) weapons are most effectively delivered from medium altitude. That is, in fact, where even the A-10 delivers them from. If you force that same pilot in that same aircraft down to low altitude, he becomes far less accurate and has a much more difficult time engaging enemy targets. You increase the chances of him killing friendlies, and decrease the chances of him killling the enemy. He becomes a less effective close air support asset.
"It seems logical that an A-10's slow speed allows the nose of the aircraft to spend more time on target than a supersonic aircraft."
Whether the aircraft is an A-10 or F-18, weapons delivery passes are planned for about a 3 to 5 second run on the final attack heading. Anything longer than that, and you increase the likelihood of being shot down significantly. For the same reason, our soldiers don't leave cover and stand exposed for long periods of time while they engage targets.
"I am also not a fan of using a $10 million plane with a $100 thousand bomb to destroy a toyota pickup truck with 3 men in the back when a million dollar aircraft with $300 worth of depleted uranium slugs would do the trick."
You are underestimating the value of aircraft (including the A-10) and overestimating the value of the weapons. Laser guided bomb units are a lot cheaper than $100 thousand. They are also a lot more likely to get a "kill" on a moving pick-up truck than a 30mm cannon. And that is the bottom line. It is not the operator's job to balance the budget. Destroy the target quickly and you decrease the chances that target will take a priceless American life.