The practical among us have always tried wringing maximum performance from ourselves and our equipment. Why buy a pickup when you can transport the family milk goat to her annual tryst with the neighbors billy in the back seat of your sedan? That's not to imply the tryst is to occur in the back seat, just the transport.
Applied to hunting, this philosophy suggests a single rifle for all reasons/seasons. Thus, the old .30-06 for North America and the .375 H&H if you plan to hit Africa. Aiming that do-it-all rifle should be handled nicely by a sturdy iron V and post until our eyes begin to fail us. Then we grudgingly pry open our wallets for a telescopic sight. But not just any scope. This must be the Jeep of all scopes, the Houdini of all scopes, able to twist and bend and conform itself to any situation from charging elephants at 10 yards to peeking prairie dogs at 1,000 yards to sneaking whitetails at days last gloaming.
Ah, but in both cases - rifle and scope - practicality leads to overkill. How many rabbits or even whitetails do you want to shoot with 300 grains of projectile generating 4,000 foot-pounds of energy? How many elephants do you want to stalk with a scope the size of a streetlight atop your rifle?
Inconvenience is the downside to using "universal" tools. It isn't always pleasant putting up with oversized equipment for small jobs in order to have adequate equipment for big jobs. Despite this reality, many shooters/hunters continue searching for the perfect, do-it-all scope - and find it. More or less.
The first requirement in this do-it-all optic is extensive zoom range. A 3x variable will magnify three times over its lowest setting, thus the common 3-9x. Start with 4x on the bottom, multiply times 3 and you get a top power of 12x. A 6x with the 3x zoom factor cranks up to 18x and so on. For years this was the highest zoom ratio engineers could wring from a sporting scope sight, until someone squeezed out a 4x zoom range in a 30mm main tube. Try putting the 4x zoom in a one-inch tube and you sacrifice eye relief and/or a full field of view. Just last year, Swarovski set a new benchmark with an amazing 6x zoom range in a 30mm tube. This means you can now buy a 2-12x Swarovski that should adequately magnify everything you'd ever want to shoot with a multipurpose rifle.
Of course, a number of experienced big game hunters have long suggested you can cover the waterfront with the old 3-9x or 4-12x or any of a number of variations in that range, particularly for big game. If you'll mainly target varmints, a 6-18x or 8-24x should do the job. Pick your poison, but watch out for shrinking eye relief (ER). Many, but not all, variables sacrifice ER as power increases. Those that do threaten to brand your brow during recoil. Aim for a minimum of 3 inches of ER at the .30-06 level of recoil, 3.5 inches with .300 magnums, and 4 inches would be better.
After power, variable shoppers want brightness. Everybody wants brightness these days. Most reach for it through oversized objectives, and those really put a strain on versatility. While a slightly higher zoom range adds little weight or bulk to a rifle, big objective lenses do. And that's the problem with high magnification scopes. In order to get 5mm of exit pupil at 10x, you need a 50mm objective. That may be fine in a stand, but you'll regret it while climbing Elk Mountain, thrashing through blacktail bedrooms or slogging through Moose Meadow. The scope will snag limbs, unbalance the rifle and become a real drag to carry. This doesn't mean you will not or should not buy one. Just be aware of what you're getting into.
Leupold offers something of a cure for too-tall scopes in its VX-7 and L lines with those concave objective bells/lenses that mount low over barrels. The weight and bulk are still there, but balance is better.
Many versatile scope buyers sacrifice a bit of window light by scaling down to 42mm objectives and compensating with superior resolution, flare control and fully multicoated lenses. All these things improve light transmission and perceived brightness without adding bulk or mass. Try it.
Yet another option in one-gun scope versatility is actually two scopes in quick release mounts. Mount a 1.5-6x 20mm for close, big and dangerous game. Take it off and clamp on a 3-9x 42mm for general work or a 4.5-14x 50mm for low-light sniping. This gives you more options plus a backup scope in case one goes down midseason.
The market certainly offers do-it-all scopes. Only you can decide if they're right for your needs.