I am firmly convinced that the two missions are not compatible. There is no question that some people can switch between the two and do both equally well (not me, too impatient for a good FID worker) nor is there any question the Groups have done that over the years -- and pretty successfully so. That does not change the fact that each mission IDEALLY would have operators that were psychologically and emotionally attuned to that particular mission.
This has been a debate in our community for as long as I have been a part of it (only since 1979), and of course there is the argument that if you can do UW you can do everything, which an argument that quickly falls apart when seriously examined.

There are several challenges in trying to produce an ideal FID force, and we seem to simply gloss over them, or come up with recommendations that probably will be less than helpful on the tip of the spear.

1. Credibility: maintaining credibility with your foreign students or counterparts can be a challenge, if you don't have real experience in the subject you're teaching. A SF Soldier who was an infantry platoon Sgt or a Team Leader who commanded an infantry Co prior to coming in SF carries some credibility when we're trying to train and advise an infantry unit. Not everyone in SF has that experience, so we seem to fall back on what we know, which is door kicking skills, which is now a skill set common to all in SF, but not always the most useful skill set in a COIN environment.

2. Language/culture: definitely value added, but only if you have the right language. I have seen to approaches in SF, one is where the entire ODA is focused on the same language, which means the team has a capability in the countries that speak that language. The other is assign different languages to each individual on the team, in hopes that whereever they deploy someone will at least have some language capability. While better than none, what does this really give the force? Let's say we're in Thailand, my medic speaks Thai, everyone speaks a Chinese, Russian, Arabic, Korean, etc. Do I really have a Thai language capability? My medic needs to teach his own classes, take care of medical issues, etc., I can't use him as a full time translator. There is still a benefit, but the traning management challenge is based on a guess (on where we might deploy). Assuming we guessed correctly, and we deploy to assist country X by conducting FID, and my whole team speaks X'ish. That is extremely powerful, at least until it is time to rotate out. How many teams speak X'ish? How do we sustain operations with the appropriate language capabilities? If it is a major operation like OIF or OEF-A then everybody needs to play to sustain the effort, not just those regionally oriented to the area, so the second order effect is the guys from out of the region are losing their regional expertise.

The point is how much effort should we invest in language? How much do we really get in return on our investment with the way we're currently employed? Change the way we're employed, then it is a different matter.

3. The other issues have to do with authorities, leadership, task organization, etc.

Far more importantly, each type of mission requires extensive training. Attempts to make Teams adept at both will unavoidably and adversely affect capability in both mission sets. Not to mention adverse impacts on operational employment...
Go back to challenge 1, how do you train to be an advisor in a skill set if you haven't done it? It can be done, we do it all the time, but what trainer is better? The one who is an expert in his field, or the one who's knowledge on the topic is text book deep, but he has the right mind set to be a good teacher? To be honest, it can be argued either way, and is highly situation dependent.

Getting back to the larger issue of DA, SR, CT, FID, UW, etc., I think it is much bigger than the individual, a lot of the capability is resident due to the unit's task organization. Obviously the Rangers are much better organized and equipped to conduct large scale DA than SF. SF is task organized to do UW from the ODA to the Group. Rangers are task organized to do DA from the squad to the Bn. And there is more to the unit than its organization that enables a capability, it is the unit's culture, its collective training, it's C4I structure, etc. We need to get over the rice bowl fights and focus on winning the fight. Assigning the right forces to the right mission makes sense to everyone, but in reality it is so hard to do.