Sheets of stamps were cut down the imperforated middle, producing a number of straight-edged stamps on each sheet. Worth less than those perforated on all sides because of their less attractive appearance.
Agree with you DaveG28, there seems to be some obsession with perforations on all 4 sides. Well-centered is (IMO) well-centered. To each his/her own though.
Ottox, looks like a pretty common used 146 or 157 though; worth around $5-10, plus or minus if you were to try and sell it. Hard to tell from the photo, but if it had a grill, would be worth $150+ as a 135.
As for the "obsession with perforations on all 4 sides" were I to come across a straight-edged stamp that I didn't have yet, I would keep it and probably put it in the main page in my album. However, if I had two copies, one straight-edged and the other fully perforated, all other things being equal, I would display the fully perforated item. Its just more attractive. Its pretty certain, most others would do the same. Hence the reason fully perforated stamps normally are more desirable.