Don't you mean having a non-seamless body makes the figure usable? Luke is in action in training, and if they did do the backpack you'd probably want him holding the straps ie bending the elbows all the way.
It's tricky because non-seamless with exposed joints looks bad (even Leia Endor, with sleeves partly covering the joints, is a bit of a problem) but yeah seamless you really can't bend the arms beyond maybe 90 degrees without possible damage.
My guess is you'd either have to forgo the ability to do that "gripping the straps" look because his arms are pretty much bent like a paper clip in that moment (i.e. HT having "don't bend the arms beyond 90 deg" in the instructions) or they'd need to do swappable arms - like with two arms that are uniquely sculpted in that extreme, fully bent pose.
But to me having seamless arms is essential for a Dagobah training Luke - so you can do handstand, the "raising the X-wing" type pose or even just a museum type pose.