I guess in the SAF concept of a 7-man section, we cannot afford
to have a designated marksman who carries a weapon that's too
different from the basic assault role.
In the old days, we have SAW gunner, sharpshooter, M203. The only
guys carrying regular AR's are the LAW gunner and the Section IC.
Depending on each section's makeup, it varies. Some have 2 SAWs,
some 2 M203's, some 2 LAW and some have 2 sharpshooters.
With the SAR-21, I may be wrong but the need for a separate
sharpshooter has somewhat been made redundant as everyweapon is now
a sharpshooter weapon.
Unlike the Russians I posted above, the SAF requirement for a
section sharpshooter is actually very low - just a 1.5x
scope.
IMHO, our section sharpshooter is most importantly a rifleman
first, due to the small size of our sections.
So if they are armed with such a specialized weapon like the one
you posted, they may not do so well in the assault role.