If it came at zero cost, then it wouldn't be a big deal. But, there is a cost-- both money and time that could be spent training to your primary mission. The Harrier guys on the boat barely get enough flight hours as it is. Throw in the requirement to train the air-to-air mission and they are stretched pretty thin. The reason I don't want the capability is because it isn't really a capability. It would be like if the MEU was carrying a squad of Marines. I guess 1 squad of Marines is better than zero Marines... but WTF are you going to do with 1 squad? Are those 2 Harriers going to do a day 1 of the war self-escort strike? Once you throw on the fuel tanks, 2 AMRAAMS, and LPOD where will the bombs go?
In general I am against feature creep, especially when the product is already pretty good. This article started out talking about A-10s. Why not throw a radar on the A-10 and give them AIM-120s too? Well, because they don't need them, and they wouldn't really be able to do the air-to-air mission correctly anyways. Same with Harriers.
I'll throw your community a bone and say that you guys have a slight edge on the Marine Hornet community when it comes to standard CAS-- especially the F/A-18D community. Sometimes having 2 guys in the cockpit doesn't make things go faster, but the opposite. The hornet has more payload and time on station, but your average harrier section is quicker from 9 line to TOT, all else being equal.