Well, each additional person raises the monsters' hps by 75%, whereas you are theoretically increasing damage output by 100% with an additional player with comparable DPS. So it should be worth it to play multiplayer as long as the players' DPSes are more than 75% of your own.
Stacking auras and team buffs and crowd control is really good in coop as well.
I got a friend that has issues with 60k dps, but when he's with me and I'm playing my semi-Crit Mass Wiz, I can almost perma-freeze all mobs and tank them with Diamond Skin and he just pounds on them with his massive AoE. It's faster for him, and faster for me.
If you can find some1 to match your damage to farm with, then co-op is nice. I have a WW barb with over 100k damage, and its very hard to find anyone that can keep up with me, and so it is much more productive for me to farm solo. I have sprint up 100% of the time, and kill elite packs in less than 8 seconds usually (and if I zerk its like 4 seconds).
On the other hand, farming solo gets boring quickly, whereas farming with a friend is more entertaining and I can laugh at them with they TP to me and die instantly cuz I'm in the middle of an arcane sentry/desecrator CF.
is there better MF or something because I lose use of my MF follower so I take a MF hit as well
Unless your follower accounts for 12.5% of your total MF, multiplayer is still more lucrative (again based on the assumption that player DPS is close to equal. Of course the number of factors MF has on drops makes the 12.5% number a little smaller (probably closer to 7.5%-10%), but still not enough to make a follower more lucrative than multiplayer.