Why encourage bad habits in new players?
How many times in the old Castrum Meridianum would the party wipe at the start because someone new to tanking went LEEEEEROOOOOOOYYYY JEEEEEENNNNKINS and ran through all the mobs, taking 2 with him as the rest started gangbanging the healers or the DPS who got off a nice AoE burst? After all, that what they saw other tanks doing when they were in there on other jobs, or what they saw being done in videos.
It happened far more than it should.
You should always make certain you have proper enmity established before moving onto the next pack. It doesn't matter that the DPS and healer will be okay with a stray mob hitting on them most of the time. That shouldn't be happening in the first place. Your role as the tank is to make certain they are hitting you instead of them so they can perform their own roles more effectively.
If that one DPS is confident enough in their ability to run ahead of me while I'm establishing enmity, then I'm not going to worry about them. I'm going to worry first about the healer and other DPS who may not be as confident. New players can panic when something is hitting on them and a lot of them will instinctively run back to where they came from instead of running to the tank.
It is dumb to treat every random party you're in as if they're a bunch of MMO veterans who know how to handle things. For many players, this is their first MMO and it can take a while for the lessons about group play to sink in, especially when most dungeons are done in total silence other than a random "Hi" at the start and a "gg" at the end. Many times I've been in parties with new players going "I was in here once before but I had no idea what was happening because everyone was rushing ahead and all I could do was try to follow".
That's not a good thing. How many times do we hear people complaining about players who don't know what they're doing at level cap? Maybe if the veteran players weren't in too much of a rush in leveling content to help those new players understand things, we wouldn't be getting as many complaints.
I know it doesn't if you know what you're doing.
How many players out there don't know what they're doing?