Why? If your raid has three healers, why do you need to be able to switch specs? It's not like you will be switching between raid and tank healing.
And further: Your itemization and reforging is different for the two specs. Do you also advocate carrying two sets of equipment around so you can "be the best you can be"?
I find your argument less than compelling. I actually have both healing specs, but the only time I use my Holy spec now is when I'm pugging a raid and they need an AoE healer - in guild raids I'm always tank healing.
OMG priests had 2 hotfixes, I can't handle it anymore!!! *Jumps in front of bridge*
Honestly, just play the spec you enjoy. I've been disc since level 10 and still loving it. Disc is really cool to play if you know the fights well. Alot of damage is predictable, thus giving disc priests a valueble roll. I love allowing other healers to spam their small heals because I counter incoming damage.
Because sometimes the benefits of PW:B will outweigh the stronger aoe output of holy, some fights also require very little tank healing. Maybe one of your regulars is not available for some reason so you need to take another role. I'm sure anyone can come up by any number of reasons, it doesn't really matter.
Ideally you may want to carry two sets yes, but at least personally I favor stats pretty equally equally between the specs (haste > mastery > crit). I change some pieces basd on regen between the specs though.