Well,it keeps it more "original" if that's a factor.Plus the POSP side rail mounts can be removed and replaced with no loss of zero,this is relatively important when disassembling for cleaning.When you use a top cover mounted scope,not only are they less than stable due to the loose fit of the top cover,but you will need to re-zero the scope every time you clean the gun.Add to that,my personal opinion is that they just look better,and therefore make the gun look better...You can find them on Ebay and/or one of the AK forums for between $90-$125 if you check frequently enough to get them before someone else does...All in all,it is a personal choice in the end...
You could always get a side rail mount with a Weaver style top,and put a cheap scope on it,though...