A disc priest will see upwards of 1000 less healing on their gheal. It's a bad spec for people too lazy to respec between arena and raids. Reminds me of the Chochsky's manager lecture about doing the bare minimum, because that is what raiding as disc is.
At the end, the DPS gains from PI and damage reduction of PS are simply not even close to enough to make up for the many major healing losses listed many times already. In other words, speccing disc will most likely cause more wipes than it prevents.