Because I've been a DM, I always seem to have an eye towards balance when it comes to classes and class abilities within the party. As a player, for instance, I want to make sure there is an arcane caster and a divine caster in the party, and am willing to, "fill the gap" with an appropriate character. This is especially true even in one-shot games, simply because I know it will help the party to overcome the most diverse set of challenges the DM can produce. It helps to keep the game moving, and that's always important.
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Currently, I am in a situation where the three other players besides myself have all chosen the leadership feat to gain a cohort. It will soon transform my involvement from 1/4 of the party, to 1/7th of the party, as the other three players will each have two characters to control, while I have my one.
Now, if I were DM, I wouldn't let it get this far, but in all fairness, I don't think the DM anticipated this at all. Nevertheless, I am left marginaized as a player unless I take the leadership feat as well for my character.
While there are only a few things that IMO, "break the game" for me, and make it unplayable, this is certainly looking to become one of those situations unless steps are taken by either the DM, or myself (by taking the leadership feat). This goes beyond party balance. Now it is about player balance. And left unchecked by a DM or other players, can result in the loss of a player.
As for party balance, it seems that our party will still lack the ability to track, even with the added cohorts. Which leaves me to pick up the slack with a potential ranger cohort. Certainly not my first choice for potential cohorts, but with some work, could be worked into the story of the game.
My resentment then, is that I feel forced to choose these things to keep the party balanced, and the game fair for me, the player. I thought this was the DM's job! Anyway, when you take the choices away from the player, as in this particular case, it can become very unfun. And, as every gamer knows, that's a dirty word!