[quote name='Jodou']They would be sitting in town just like they do now. The difference being whatever companion you have selected would still apply their buff(s) to you. I'd have friends join my game and

me over by accident because my companion would take off and I'd lose my armor buff. When something like that becomes mandatory, it's just another reason to play solo.[/QUOTE]
you realize the IAS buff from the enchantress has a range of 40 yards...if she's down for the count, it's likely you're not close enough to her or it's just an active skill mechanic vs. a passive one. If she's incap'd often, all the more reason to stack Vit on her. My templar has 82k hps. Belial can't kill him, he only dies when a bunch of accursed death proc poison around him for more than a few seconds.
Maintaining the follower buff in MP is not issue, your friends joining your game w/out warning is the problem. I'd advise turning on the invite only feature or telling your friends it's mandatory to give you fair warning before jumping into your session.
...but let's entertain your idea for a moment.
check this out. you have a decent geared barb w/shout, monk w/the armor mantra, and a wizard w/pris armor. they all buff up, your armor is likely going to be near 13-15k, the wiz is probably around 18-20k. You want to add the enchantress follower buff to that too? That's a bit unbalanced wouldn't say? especially with the dmg reduced to all elites as well as their hps.
The patch is encouraging and making cooperative play easier and more rewarding. we really don't need any more static buffs, paragon levels is going to give us inherit buffs across the board. Permanently. It's your choice to go solo or multi and utilize the benefits of cooperative play or the follower(s). There are clear advantages for playing solo, but at least to some cooperative play can be a lot more rewarding.