That's true. The MR-8 is pretty stiff in that it works only one way.
If you read the sales info, you'll see it all repeats the one description of how it works.
See the Musician's Friend info or any info from any website and Fostex too.
It all says record to 1 thru 4, mix to 5/6 then record again to 1 thru 4 and mix 1 thru 6 to 7/8. And the effects are only available when submixing the first set of 4 to 5/6. The effects knobs are only on tracks 1 thru 4.
I don't remembe about the mastering effects, but they may only work mixing to 7/8, but I'd have to check that agin, it makes sense tho...
So if you want other effects, you have to mix thru them, or use on recording. It's an inflexible algorithm, one way or the highway.
If you have a PC w/ software, then you open up your options, or if you have outboard mixing gear, mixers, effects, and a DAT or other recorder to mix to....
On the other hand, you can't forget how it works or be confused by options, etc.
Also, the files making up tracks 5/6 are still 2 mono files, just played together, and the same with 7/8, untill you convert 7/8 to a stereo wave file.