I know people will make fun of me for not actually doing anything, thus I should be stupid and not know anything, but...
The SDB uses a much more destructive compound (I don't remember what it's called), than the old Mk 80 series. Combined with the new compound and its greater accuracy, it has the same capability of an Mk 83. You add a few more DMPIs to a target and you can kill large structures pretty well. Its just like nuclear weapons, 10 100 kiloton warheads spread out around a city are significantly more effective than one megaton bomb placed in the center of the city.
In other uses why the heck would you need a 1,000# bomb or a 2,000# bomb or even a 500# bomb to take out a vehicle if something much smaller will do just as well?
Furthermore, it has greater hard target penetration capability than the Mk 80 series. Not as good at the BLU-109, -110, and -111, but still very good.
Additionally, with the strap on glide wings, each SDB has the capability of flying 60 nm when released from the Raptor's cruise speed of Mach 1.5-1.8 and 40,000 to 50,000 ft. Other aircraft will not give the SDB that kind of range, but they can give it a good stand-off range for specific types of targets. That type of stand-off comes in important when you are trying to take out those SA-10s and -20s.
They are also looking at including a datalink and the JDAM kits (to include the SDB), which will update the GPS coordinates during flight to allow accurate attack of moving ground targets and switching of targets in the air.
Of course the SDB won't take over every role. It has its place, just like the Maverick, HARM, SFW, GBU-28, other LGBs, and ballutes have theirs.
When the -117 leaves, you loose the capability to drop 2,000# LGBs. There will be situations where that capability is needed.
When the Raptor takes over, you have the capability to fly in much more dangerous airspace, have a self defense capability against air and ground threats, have significantly greater speed (allowing it to take out the targets faster than the -117, great for time sensitive targets), and perhaps the most important of all--the ability to fly missions 24 hours a day. The downside is you are stuck with using GBU-31s or SDBs. In most cases, those will probably work out pretty well. In some others, not. Overall, the Air Force is gaining a significant combat capability with the Raptor--something the JSF can't come close to touching in regard to internal payload, speed, maneuverability, and all aspect stealth. But saying the AF should ditch the Raptor for the JSF, or the JSF for the Raptor is just as ignorant as saying we should ONLY use F-15s OR F-16s.