If it's not Extremes, Savage, or Ultimate, you don't need to watch a guide, though. It takes next to no time for someone to point out the few mechanics that one would need to look out for in a dungeon boss if someone asks for it. The fact that someone asked should be enough to get someone to help, since most people don't.
OP's entire argument is that players should watch guides for EVERY. PIECE. OF. CONTENT. Including dungeons where explaining the important things would take less time than typing into a YouTube search.
EDIT because posting limit (Can we do away with this? Seriously?):
I think a lot of people in this community forget that not everyone learns how to do mechanics the first time around - or that people have different learning styles. Some people can pick up what to do by watching other players do the mechanics for them (i.e., visual learners can learn from videos), but others have to learn by doing. I fall in the latter category, and I'm sure there are plenty of others that do as well. Not everyone can pick things up as quickly as you or I could either, so some need to learn some patience when dealing with players like this - if it's their first time and they're asking what they're doing wrong, I have endless amounts of patience for people like this. I'd rather someone ask me for help if they don't know what they're doing as opposed to staying silent, but people like the OP that brush off helping others with "watch a video" is one of the reasons new players have said that they don't bother to ask for help. Answering someone's question of "Is there anything important I need to know here?" with "Go watch a video" is not helpful to them; it's probably the least helpful thing you could say to someone in the midst of a piece of content. Taking 10~15 seconds to say "Yeah, watching out for Crush Helm - that's his tankbuster, so make sure to pop a CD for it" is much faster and far more helpful.
If you or OP were talking about Extremes, Savages, or Ultimates, you would have a point. People should go into that content with at least a little bit of preparation - even if they're joining learning groups. But dungeons? Normal Trials? Normal mode raids? 24-mans? These can be explained fairly quickly and even on the fly if necessary, with very few exceptions (and even those exceptions can be cleared if the newbies listen). Sure, they may screw up, but they probably would have done so even if they'd watched a guide beforehand.
If you get a player that won't listen, well, they aren't worth the effort and it's unlikely that they would have looked up a guide, anyways.