Yes, as long as they don't give you competitive advantages such as x-ray or the ability to see through walls in PvP.
I did this ONLY ONCE IN THE PAST maybe 6 months ago X-ray resource packs don't work well at least the one i used ALONG TIME AGO. Also not implying you guys to try it. It was just a simple test.
no I was just going to use 1 that makes blocks look more smoth ie grass will just be a constant color to me.
Actually any x-Ray pack would not work anymore because of a change in the way Minecraft renders non transparent blocks in 1.8