For magnification of 10x or lower, or for shorter ranges like 100yds, adjustable objectives are unnecessary, and introduce complexity into the scope that could compromise the gas-tight seal .
If you desire to shoot at much longer ranges, with higher power magnification, then an adjustable objective may be a benefit.
The purpose of an adjustable objective lens is to correct for parallax vieiwing, which causes the reticle not be exactly aligned with the true point of aim, when viewing thru the scope.
In most cases, if you align your eye perfectly centered behind the center of the reticle when you shoot, you will not experience parallax problems.
In most quality scopes, even without an adjustable objective, it has been calculated that it only amounts to about an inch at 1000yds for parallax error.
Adjusting the actual objective lens for parallax correction is more optically efficient.
Side focus introduces another lens on a suspension system, directly in the line of transmission, and could be considered less optimal. Also, the side focus has an additional hole in the tube,which could compromise tube strength and integrity. However, the side focus is easier to reach from the shooting position, and is very handy.