Motion Blur, like most things Blender, was likely coded by someone who needed a particular effect that Blender didn’t provide at that time.
Rather than calling something that you didn’t have to pay for “stupid” you always have the option of joining into the fray, coding it the way you personally like it, patching your build, and submitting the patch for inclusion in a future build (so that we all can have the benefit of complaining about your work like you’re complaining about someone else’s).
If you don’t know how to code and aren’t willing to learn then you are: A.) going to have beg someone who does know how to code to do the work for you ((not likely to happen if it’s not in popular demand…you can request editing rights for the Blender wiki from LetterRip and upon receiving them you can then update the feature requests section)); B.) going to have to pay someone to do it for you ((again, not likely as you’re already complaining about “stupid” freeware)); or, C.) you’re stuck until someone who can code the feature needs it for their own personal task and decides to tackle the job.
From the wiki: “Bf: or Blur Factor defines the length of the shutter time as will be shown in the example below.” See: