So from what I can find to read on the subject, weld-thru primer keeps
corrosion from happening, so couldn't I just use metal ready instead? According
to the metal ready lable, you can weld to areas that have been 'metal readied'.

Getting ready to weld patches in, both using butt-welds, and welding
layered pieces together someone please insert the correct term here,
I already feel like an idiot).

Just wondering if the weld-thru primer was necessary if I
coat the back side of the pieces I'm going to weld with metal ready for rust
protection (since I won'tbe able to get to them after the welding is done).

Thoughts? Lambastings?

Metal ready is more of a treatment / rust converter IIRC. Good idea. The primer is a coating on top of that. IMHO, it's a good idea too - some of it will burn off right around the welds, but any little extra layer of protection against rust is good.

Hopefully the areas you are covering / welding up will not be getting exposed to moisture any more. The main thing is the treatment after the welding. That's when a good treatment and paint will seal it up and stave off the rust.