I don't think they're essential, as long as your OS-level color management has an advanced mode for optical calibration (this is a checkbox in MacOS; not sure what/where in Windows.) The eye is quite a good comparator, so visual calibration can give you very accurate results as long as you use enough data points, which is what the advanced mode does.
On the other hand, if you're not very confident about your ability to compare colors, or just don't want to spend half an hour every month or so going through the visual calibration process, a hardware calibrator may save you enough time and stress to be worth the money.
Incidentally, the lower-priced monitor calibrators ONLY calibrate your monitor -- you won't have a fully profiled system unless you calibrate your scanner and printer as well. This requires additional software, as well as precise targets that can be scanned, printed, and then measured to produce custom profiles.
I'd say these full-workflow calibration systems probably aren't worth the expense unless you've got money riding on the accuracy of your color output -- for example, when we switched at work over to an all-digital workflow, we forced our main photographer to implement color management and supply the profiles to our engraver, so the photographer's proofs would be consistent with the engravers. Color proofing and color correction are significant expenses in this type of work, so the cost to the photographer (probably several thousand dollars) was a worthwhile expense, especially since it helped him keep an account that bills several hundred thousand dollars a year to his studio. But outside a commercial color environment, I'd say this would be more than you need to do.
