Ok...homer
Here's what u should know.
YES the use of the boards between down n flt detents is NOT recommended as procedure per se.Most instructors(myself included) would not just discourage the use of intermediate positions,rather go a step further and ban it altogether.
Reasons are quite simple really.As someone mentioned earlier,yes...a) Assymmetrical deployment during turns with the boards in an intermediate position could dramatically increase the roll rates.Especially since the fact remains that speed brakes are most effective at higher speeds,therefore i am assuming that thats the flight regime in which you are using them.This higer airspeed would positively aggravate the roll rate. And then....b)The 737(classics and ng's unfortunately)have a peculiar design feature in rgds to their speed brakes.Which is NOT the case in almost every other aircraft that I have flown,744 & 777 included.When u deploy the boards in the air,the ONLY time the flight spoilers are in a locked position is when they are in flight detent AND in ground detent(i'll come to that later).Therefore in ANY other position they are suceptible to the now infamous flight contrlos 'flutter'.This combined with the eddies created over the boards in the high speed regime and further combined with the aerodynamic forces during a rate one turn at those speeds could really become quite a dangerous flight situation.Also,during manufacturer's testing in the early days,the drag values had only been calculated for the 2 'fixed' positions.Because in the other variable positions of the speedbrake handle,the exact angle of flt spoiler deployment was not known(or at least....it varied with different aircraft of same type).Thus the inability to calculate performance figures.Its either all or nothing with the ng speedbrakes.Not so however with more advanced airplanes with Fbw systems like the 777 for eg.Here you can and SHOULD use any and all intermediate positions depending on the drag required.AND they have done away with the 'flt' and 'ground' detents too! Of course that's just evoloution for you...
The 'ground' detent CAN be used and WILL deploy the ground spoilers in the air too(if i remember correctly,a single failure could deploy the ground spoilers in the air if the handle IS in the ground detent).This higher drag value could be catastrophic.Especially in the high speed regime.
Buffeting of the wing during normal use of the speed brakes is a normal phenomenon.And should not limit the use of the boards.Provided of course u do not exceed the manufacturer's 'recommended' values.
Well...i tried to help.Hope i succeeded.
A very good question though.Controversial....at the least.