Stems

Stereo mixes of groups basically.

They are useful for mastering guys so they can adjust or tweak group levels. I like doing this because their listening environment is far better than mine and most engineers, so if they want more lowend they don't have to use eq, but rather tweak and adjust the bass stem. I always let the mastering guy know what I want maintained as far as the mix as a whole goes. I've always loved doing the stems if the mastering engineer prefers them.

Stems are the files mastering engineers want when they think your mix sucks.

Stems started with film mixing. Different instrument groups broken out so a director can nuke something that's getting in the way of dialog. Then music guys stole the idea and name and here we are.

In my experience, they are more often used for a quick "remix" than for the mastering guy. Personally, I don't want the mastering guy remixing my mixes, so they don't get stems unless there's a catastrophic problem.

As for master bus FX like compression, EQ, etc., yes, doing stems precludes this. UNLESS you monitor and mix to stems, then bus them to the stereo bus with processing, and print both stems (uncompressed, EQ'd, etc.) and compressed 2 bus stereo mix simultaneously.  IMO, I always do that when doing stems. Stems are for screwups, fixes and "I changed my mind, can you bump up the vocal 1dB" scenarios - not for final delivery, and I want my 2 bus mix the way I want it so....yeah always print the 2 bus mix.

I don't know…. I don't think if a mastering guy prefers using stems it qualifies you as shitty mixer…… I think that sometimes mastering guys have a way of doing things and some prefer stems. I use Hank Williams now and he's a guy that only masters the two mix, but he blows me away every time I get something back. Now, what I like about him, is if the vocal/bass/snare needs to be down or up a little he won't grab the EQ and start notching and doing it himself. He'll shoot me an email and ask me to make those adjustments. If he did use stems I think he would be a guy that only used them for that purpose. But, I do agree with what others have said, there are some out there who will do some really dumb and shitty sounding stuff to your idea by using compression on the stems.

Everytime I've sent stems to any of the guys I use besides Hank, they always just use them for level adjustments or if there is an issue they can fix it so it's much faster than having to get me to go in and make those small changes.

So, that's why I'm not totally against it, and sometimes if a mastering guy feels he can knock your socks off by requested stems, send the dude stems. If it doesn't, tell him to master the Two mix you sent up as well 8p

I often think of mastering as being part mind reading. What did my client hear in their mix room? What would they change if they were hearing their mix on my high resolution monitor system? I'm always looking to correct unintended problems. I start with the assumption that the mix is exactly what my client wanted it to sound like and I never want to step on that intention.

You will be required by any major label, and most indies and self financed projects, to deliver stems for the purpose of REMIXES, as in dance/EDM/techno remixes. It's a big market these days and for the last 5 years or so it has been a part of every single mixer agreement I've signed. It's in the paperwork. You don't get your back end money if you don't deliver stems.

Another great use is to eliminate a TON of dsp drain. If you have 20 drum tracks and 4 or 5 busses used to process them and 20 or more plug ins running on the drums, in a mix with 100 tracks you might find yourself running out of processing power. The fix can sometimes be to print a stereo drum stem and deactivate all the tracks/busses/plug ins associated with the drums.
Run computer...RUN!!!!

I, for one, would NEVER give a mastering person stems unless the person in charge of the project insisted. And even then I would try and fix my stereo mix first. But at the ME's request? Nope.

I should add that if this is an electronic or hybrid electronic artist they may want stems to use for playback in their live show. I always always always print stems. Every single time.