Sharp 7-9" diagonal cutters (Klein, Milwaukee, Knipex, Felco, et. al.) are sufficient for cleanly cutting bowden (not braided) steel cables found on bikes. Cutting farther back in the jaw, straight across, and with no tension on the cable will greatly lower chances of fraying. Coiled brake housing is also easily cut with diagonal cutters. Basically, the only thing that wire rope cutters (as they really should be referred to) are necessary for is expediting sizing index compatible housing (which can be done, albeit slowly, with a rotary tool/cutoff disc), and cutting braided steel cable, which are quite uncommon now, but were found on older bikes.

Speed is the biggest advantage. It's easier to grab a different size, and quite fast to spin down a fastener. I would consider them a nice thing to have readily available, but far from necessary. If they aren't front and center in your work area, it is unlikely that you'll go out of your way to use them. The only fasteners on a bike that a ratchet is the first thing I reach for are axle nuts. Quite simply, with the exception of self-extracting cranks, almost nothing on a bike will be more easily removed with a ratchet. Corrosion that is causing difficulty removing a bolt may cause me to reach for a ratchet, but they are usually such low force applications that the ratchet doesn't trip, so the tool essentially becomes a large, unwieldy wrench. Thumb ratchets obviously fix this problem, but it is still almost universally easier to use the right sized wrench, and just grab the right size for the part you are working on. Specifically re: hex keys, ball-end keys in the sizes used on bicycles are easy to spin a fastener down one handed, so T-handles become much less necessary.