The sixth slot is just there as a bonus. You'll never know what people want to do with it. I could imagine many useful situations where a sixth slot might help.
For example, you need someone to host your map, but this guy doesn't really want to play and is AFK the whole time. You can just make him host and everything is fine.
Also, maybe there is someone who wants to do some other task while the others wanna venture dungeons? You can do so. Or you just have one guy being afk ... you always have.
Warcraft III is not the most stable game ever. It happens a lot that people simply disconnect during the game and having to rehost everytime that happens is a pain.
Dungeons are balanced for 5 man groups.
About the issue of the "noob party" that plays the map without knowing about the limit: The first dungeon is not limited. You can have all 6 players inside.
Also, you can 6-man world bosses (which is fine, as world bosses are intended to be easier than dungeon bosses) if you wish so.
About the Shield bash issue:
As Muzzel said: This is some kind of trade and depends on what you want to do with your Squire. Shield Bash generates a lot of threat and with all the new Crusader skills coming, you have two options to specc your crusader: Go full strength, grab the additional armor penetration and be superior in single-target tanking and damage or go int, lose some of your damage, some threat on the single target, but have much more survivability and AoE threat capability.
It's about how you want to play your Crusader. That's why the 9 spell limit will be there in the future ... to give players a choice to play the class the way they want it.
Also, remember that Berserkers can use shields too and don't need to rely on Int. For Berserkers trying to offtank, shield slam might be a useful skill. It all depends on what you want to do ...
Fair enough, those are some legitimate reasons for the 6th slot instead of the crap I've been reading so far. I still find it an issue for newer players, and disagree with the idea of a single person doing other things ingame, seems pointless. Perhaps in the future, as soul suggested, make world bosses challenging enough to be managed by 6 players, encourage the use of a pool of classes, one from each if possible if the 6th mail gets implemented as an offtank/disable. Currently the fact that its real only bonus is an afk host does seem a bit weak.
I wasn't aware that the first dungeon was a 6 man, see, that rendered that point I made irrelevant, I like that, that helps newer people get into the early game and get used to the system. I'm unsure but is there anything in the F9 area that covers the dungeon limit so people know and understand from an early point (if they bother to read the info) that later dungeons are made for 5 man runs?
Of course I'm bloody angry water, I expect better from this community, being able to discuss and sort out issues with players is a must, If I'm wrong, if they're wrong, if a compromise or an idea can be met and worked with, that's what these forums are useful for, perfecting the game we love, when people are straight up like NO it doesn't help.I get stressed easily when I have to deal with the same crap I've dealt with through previous experiences with online communities, thus are the woes of the internet.
You won't see zerkers with shield bash, highly unlikely they will use it with the number of skills they have that rely on two handed, what's the point. It's like crusaders at the moment going full int for crucify damage and neglecting all other skills and ap, its pointless.
Currently, zerkers have a backpack that is usually either full or close to the limit because of the items that cover their fire res, urn, and I think a few others, I'm not sure I don't run a zerker. Perhaps instead of working a class based off trophies and two handed weapons into an offtanker with a backup shield, try fiddle with this 6th class and see if it can manage a solid offtanking role with a shield, or some kind of bait, something to pull agro, generate it faster or slower depending on skill set ups, things like that.
Unless you'd really like to see zerkers running around with shields for one skill out of their set in dungeons, which again, seems pointless.
I'm also kind of curious as to why a class like a zerker can use two offhanded items where a crusader can only use one, I'm fairly sure the same happens with sins/bards and the like. Is there any way in the future we can work in, if you have your heart set on this shield zerker idea, shields for zerkers that drop from bosses, blues, greens, whatever. Not zerker only shields but a more average shield than a shield that is set for 'Crusader Only'. It doesn't have to be a so called legendary item but I imagine it would have more int or so, a shield that allows the crusader to do what it does best, generate agro, hold the line.
Sorry for the massive wall, just trying to cover as much as I can
I appreciate the calm response zwieb, I'll try and tone down my angry posts.