Do the constant aperture lenses make a practical difference for low light shooting, in aperture priority mode or leaf speed? If the aperture changes as it approaches with a variable aperture lens, the camera can always compensate by varying the shutter speed and / or ISO, then does it make a noticeable difference in practice, to shoot in low light?
Many times, when shooting with limited light, you are using the manual exposure mode with the largest aperture and the slowest shutter speed possible without the camera being blurred or out of focus due to the movement of its subjects. When using a constant aperture zoom lens, you do not need to change the ISO to compensate for the aperture change as you zoom. Many cameras allow the use of automatic ISO in manual shooting mode, but some do not. Even with automatic ISO, the way some cameras handle the "partial stop" ISO setting (ie ISO 125, 160, 250, 320, etc.) makes them less ideal than the "full stop" ISO settings ( that is, 100, 200, 400, etc.) In such cases, the preferred way to use Auto ISO is with "full stop only" ISO values enabled, which means that when a variable aperture lens moves through maximum openings in increments of 1/3 stop, the exposure "bounces" up and down. So I film regularly there is a world difference between a constant aperture zoom and a variable aperture zoom.
Would you get a better quality lens for the same price if I don't insist on a constant aperture?
In general, constant aperture zooms tend to have better optical quality than variable aperture zooms. It is not so much the inherent quality of a constant aperture design, but it is an indicator of what the market demands for higher priced lenses. Usually, but certainly not always, compensation is not a better optical quality in exchange for a variable aperture design. Rather it is a variable opening in exchange for a cheaper price.
And, taking a step beyond the F4 constant aperture zooms, how do the F2.8 constant aperture zoom lenses compare? Are they generally as sharp as premium lenses? In other words, if I don't need a wider aperture than F2.8, would a lens with constant aperture zoom F2.8 replace multiple main lenses at its focal length?
All that depends on the particular lens. The Canon EF 70-200mm f / 2.8 L IS II is one of those lenses that can respond (a qualified¹) yes to your last question. Some other zooms are very close or equal to their main counterparts in the same aperture configuration. But there are other zooms whose image quality is well below their main or narrower opening counterparts. The Canon EF 16-35 mm f / 2.8 L II is one of those lenses in this category. The EF 16-35 mm f / 4 L is better in almost all common aperture and focal length combinations than the first two versions of the 16-35 f / 2.8 and is also much more affordable. However, the recently launched EF 16-35mm f / 2.8 L III is significantly better than all previous Canon 16-35mm zoom lenses. It is also much more expensive than the others.
Roger As Roger Cicala has pointed out more than once in his blog series on lensrentals.com, the variation from lens to lens among all zoom lenses, even the most expensive ones, is much greater than even among the main middle-grade objectives. . Therefore, a "good" copy of a very good zoom, such as the EF 70-200 mm f / 2.8 L IS II or the EF 24-70 mm f / 2.8 L may be quite close to the I.Q. to prime lenses in their focal length range. But the less measurable characteristics of either of the two lens designs will vary from one to the next. How out of focus areas are represented, for example. Even two very different main lenses, such as the EF 50 mm f / 1.2 L and the EF 50 mm f / 1.4 differ more in things that do not appear on a flat test chart than in absolute resolution, geometric distortion , astigmatism, coma, etc.