Just wondering what the goal is.
You offer free mixing...and maybe you get a dozen or so people who are in need of mixing help that bite, because FREE is a big seller in the home rec world.
So then you mix a couple dozen freebies...and you post them on your website as "clients" you done work for...and that's the "portfolio" that gives you some kind of cred when later on when you start charging $$ and the paying customers show up and see the "portfolio" and assume that you're an in-demand mixer with all those clients.
One problem (like you already touched on)...if you get a bunch of newbs with really shitty tracks looking for free mixing...well, you know those mixes are going to be shit, for the most part, and for them to be of any "cred" value...you end up working much harder to get them even decent sounding (something your future paying clients will not know...how hard you worked to polish the turds)...so it's not as simple as doing freebies to build a worthwhile portfolio.
Of course...you can keep offering freebies, and just pick the ones that are good to use in your portfolio.
It's a way to get there...
I don't think it's a bad thing...I mean, if the mixing is good, and its free...there's no real downside, but kinda has a tone of desperation to it...
"I am really needing a large portfolio"...as though someone went into the whole studio thing too deep, and now has to generate the work to justify it...etc.
Maybe it's something else driving it...but it does have that anxious vibe about it.
The part that puts me off...is that it's all anonymous...without any link to a website, etc.
Most guys at least show some basic "studio" webpage and a bit of info if they are doing it as some kind of business.
At least then it looks like you're open for biz...and willing to do some freebies to get things rolling.