.30 US Government of 1906, 7.62x63, 30-06 Springfield, Thirty Ought Six, Ought six, Thirty O Six.
Or however you call it, is, was, and will continue to be one of the most versatile, and therefore used cartridges for hunting North American big game.
Why?
Because it uses bullets of suitable mass and diameter, driven at suitable velocities, providing suitable trajectories, while producing recoil in an average weight rifle that the average person doesn't find objectionable, all the while typically being produced in a rifle, when set up with optics a package which seldom comes in over 8.5 pounds.
Let's break down this last paragraph.
What does it mean a bullet of suitable mass and diameter?
This means the typical bullet used for hunting (150-180 grain) is heavy enough when travelling at speeds the cartridge will develop, to result in desirable penetration and expansion on game animals, without falling to pieces or failing to expand. In the simplest term critters seem to die effectively when shot by such bullets.
What is a suitable trajectory? This simply means a hunter can utilize a maximum point blank range style of zeroing, with the cartridge when using typical hunting bullets, to extend the minimal hold over distance to that which the average shooters skill set allows. (300-350 yds) factoring in optics, shooting skill, and range estimating abilities.
What is meant by objectionable recoil?
Through many ramblings and writings of the past, the recoil energy value of approximately 20 ftlbs, has since become the value in which it is felt the average person(male) tolerates recoil, without developing bad habits such as flinching. Of course stock design, recoil pads, and the individual will skew this value, but let's take the average off the shelf production rifle chambered in 30-06, loaded with 180 grain bullets(2700 fps, 57 grain powder charge), scoped up, with an total set up weight of 8 pounds.
If we utilize an online calculator for recoil such as here:
http://www.handloads.com/calc/recoil.asp
You'll find that the recoil energy is almost bang on 20ftlbs.
Why does this matter?
Because if the shooter finds the recoil objectionable, they tend to practice very little, and develop bad habits such as flinching, therefore making their abilities as a rifleman and shooter less effective.
And finally why is the rifle weight important?
Seeing as the typical battle rifles which where of a bolt action configuration usually tipped the scales at right around the 9.5-10 pound mark, and well not too many soldiers were in capable of lugging their No 4 Enfield, or 03 Springfield around in a combat setting all day long, it's easy to see shaving a couple pounds of weight off would be pretty functional for the average weekend hunter who perhaps has a fairly sedentary life most days. Yes lighter rifles now exist, but then the recoil value will increase, and the abilities to hold the rifle steady begin to diminish as well. Lighter bullets, of better construction can be had, but let's face it a 7 pound 30-06 all scoped up is danged light and much lighter is far from what most hunters will be able to afford.
So to the OP.
Why do you question the 30-06 as a sheep rifle?