You don't need it. The pack.mcmeta is used by resource packs to provide info on the pack similar to the mcmod.info file for mods. I am not all that familar with why we get that error however it is not needed. I think the reason why we are getting the error is because mods, texturepacks and soundpacks are all sort of becoming the same thing and will probably be treated as the same thing once the minecraft api is finished. I know a few other authors include a pack.mcmeta but it's not required.
I guess that it currently is not an enforced requierment but that it may become one at a later date.
I'd wager that the forge team wants us to use the mcmeta's and has there reasons for doing so, so I guess for mods we plan on releeasing that we may as well spend a few minutes on creating such a file
Okay I am having the same problem in magic launcher. I know my forge installation is correct and 1.7.2 doesn't seem to have this problem. I have not for the life of me have found a person to help me, and I would really like to play some Thaumcraft/Adv with Biblio and Biomes'a plenty tonight.. Who would like to help me??