Studio monitors are very practical, not to mention an all-around wise choice. The two foremost reasons in my mind are:
1) As an engineer, you NEED to be able to hear exactly what's going on in your audio... no more, no less. Studio monitors gives you this ability - depending on what grade of monitor you purchase. Consumer payback systems cannot give the accuracy in imaging, resolution, frequency reproduction, and power that a good studio monitor will provide (neither can most "budget" monitors)... not to mention the myriad of other technical benefits (harmonic distortion, dynamic range, transient response, well designed crossovers, consistent performance at low/high levels, etc.). It is also worth mentioning the importance of room treatment - but that's a slightly different topic.
2) The middleground is a wonderful place to be, my friend. A good set of monitors, in a well treated room, is important to an engineer for the same reason that a carefully calibrated screen is important for a film colorist. As you well know, there is a nearly infinite amount if variance in sound playback systems. The same is true of the difference in color and dynamics calibration between screens. One emphasizes reds, another emphasizes blues. One is oversaturated, while another is very bland and desaturated. The blacks and whites all vary from screen to screen... so on, and so forth. However, when working on a perfectly calibrated screen, one is able to find the perfect balance between colours, hence any given aspect of the coloration will never be too far out of balance on any given screen - regardless of its particular characteristics. A perfect skin tone (reds) will never seem too red on oversaturated screens, just as it will never look too bland on desaturated screens. All that to say, the same is quite true for studio monitors. When the creating and decision-making occurs from a balanced, middleground position, it stands a far better chance of proper translation to systems that are out of balance. It also takes the guesswork out of things, prevents over/under correction, and enables intelligent mix decisions.
Now studio monitors are not all equal. To achieve the level of imaging, resolution, and power that is needed to reproduce the more subtle nuances of your mix, it requires more than something like a Yamaha HS8. You are looking at something more in the class of the Barefoot MM27's.
Granted, while it is possible to produce great mixes on nearly any speaker, by taKing the time to really learn them; one would be hardpressed to beat accurate monitoring.