There was a time when the Monster of the Week was the standard format for genre television.
There were exceptions and special cases of course.
TakeQuantum Leap, for example.
ButQuantum Leapsimply couldnt work as a series if every episode was about something deeply personal to Al or Sam.
Some shows combine story arcs with monsters of the week very effectively.
Theres nothing wrong with any of these formats and they are clearly the best option for some shows.
There are also practical advantages to this form of story-telling.
Ultimately, like most things, it all comes down to personal preference.
Theyre not completely without merit.