As a guild leader myself, I appreciate the efforts that my players go through when they learn and gear up additional specs on their own. Tanks who can DPS and healers who can tank and other such combinations make my life and my job incredibly easy. I don't have to "train" additional players and get them acclimated to our guild way of life. I don't have to go out and recruit a specific role if another player in the raid can do it with a simple spec switch. It doesn't hurt as much whenever someone decides to take a leave of absence, because we have players who can easily cover for them. Anyone who can play two roles essentially doubles their level of usefulness to the team.
There have been cases in which raid leaders have asked shadow priests to switch to a healing role simply because they didn't have enough healers. In my guild, I have the opposite problem; I see if any of my healers are willing to switch to DPS because I have a slight excess number of healers.
If you're ever going to ask a player to switch, you need to break it down to short-term and long-term windows. If it comes down to it, I think players are more receptive to switch roles for a week or two. This gives you enough time to find and recruit someone to fill in that slot.
A skill worth learning
I think shadow DPS is a great skill to pick up -- and not just from the additional value you can provide. Shadow is a style of play that's fairly calming. With all the DOT spells and the channeling, I find it a pleasant change from something like a retribution paladin or an elemental shaman. Not to mention, the gear picked up for healing can easily double as shadow gear, with a few reforging tricks. It may not be 100% optimal, but it is quite serviceable. Unless you're at the top of progression, you can work with what you have and still get the job done. When I play shadow spec, I know I won't be cracking a top 10 parse in my raid group, but I know enough to still do the necessary damage.
Something I've noticed about healing priests who play shadow is their common ability to read the situation and what's going on. If a healer or two dies, I've seen that same shadow priest stop DPSing and just start dropping Prayer of Healing bombs (or other healing spells) to keep the raid going until the dead healers are back alive again. Sure, it cuts into their DPS, but at least they're trying -- and they're making up for it in other areas.
If you want to get started, look no further than WoW Insider's own Fox Van Allen and his Shadow Priest 101 post.
What about what players want to play?
Yeah, it does matter whether or not players want to play as DPSers. I learned this lesson a long time ago. You pay a really heavy price when you force a player to do something that he's not willing to do. Not every priest wants to go shadow, and not every shadow priest wants to resort to healing. Maybe they're just not wired for it in some way.
Bottom line: It'll be an incredible boon to your raid group if you can and want to pull off being able to DPS. Don't sweat it if you can't. And if you're thinking about forcing a player to switch specs, make sure you impose a time limit so that they can revert back at the end.
Discipline favored over holy?
I know there's big perception right now that discipline is way better than holy. I prefer being a holy player, myself, but I've been asked recently to optimize and respec over to discipline for the sake of progression.
I can't deny the data I'm seeing showing that discipline is making some fairly large absorbs. I attribute that largely to the modifications made to the crit rating when 4.2 came out. Contrary to popular belief, I don't think holy is quite dead. The distinct lack of cloth drops with spirit from Firelands is a little painful and means that holy players have some work and catching up to do. It's not their fault, though. Call it a hunch, but I suspect that in a few months, holy will get right back up when they have the additional gear. (Not making any guarantees, though.)
Another thing that's granting discipline the nod over holy is Power Word: Barrier. Holy doesn't really have a raid-wide 3-minute cooldown spell like discipline does. There is Divine Hymn, and then there is Guardian Spirit. The counterpart to Guardian Spirit is Pain Suppression, but the fact that there isn't a holy counterpart to Power Word: Barrier means that discipline has a slight leg up.
During the early weeks of new content, raid leaders are always going to want to optimize and bring as many "outs" and cooldowns as possible. One suggestion that might work is folding a talent into the holy tree where the cooldown of Divine Hymn gets dropped to 3 minutes (but the healing component gets nerfed and the healing bonus it provides to incoming spells also gets a modification). But that's just an idea. I'm not sure if there are any plans to buff priests at this point.
I'm hoping we can sort ourselves out in the next month or two, but I won't be holding my breath. I raid in a 25-player environment. At the moment, I'm not convinced that holy is on the verge of dying and is in emergency mode or anything -- at least, not for what I'm doing.
Spiritual Guidance has the inside line on pre-raid gear, valor point and raid gear, and healing strategies for bosses such as Atramedes and Chimaeron. Newcomer to the priest class? Look into Discipline Priest 101 and Holy Priest 101.