While a microscope provides greater magnification than do loupes, a microscope limits a surgeon to a smaller field (area of coverage); and as magnification increases, depth of field decreases. (Depth of field is the spatial "thickness" of all points that are in focus at one time.)
(1) For a vasovasostomy or "VV" (reconnection of the vas ends where they were severed during the vasectomy), loupes provide not only
(a) excellent magnification (up to 6x, which makes a vas tube appear to be as large as a pencil), and
(b) depth of field for the connection of the vas ends, but also provide
(c) magnification for all other phases of the procedure (making the incision, cauterizing blood vessels, and securing the vas ends) and for whatever the surgeon is viewing outside the operative field, such as reloading needles into needle holders, finding the small bits of suture created by trimming the tails of tied knots, and so on. That is, loupes and a headlight provide magnification and illumination where-ever the surgeon looks, not just at a small field as is the case with a microscope. Microscope users who take photos under high magnification power often perform the VV under lower levels of magnification easily attainable with loupes, because they see a larger field with more elements in focus at lower magnification. In addition, the loupe user can provide continuous fine focusing by simply adjusting the distance between his eyes and the subject without need to manually change microscope focus settings.
(2) For a vasoepididymostomy or "VE" (reconnection of the upper vas end with the epididymis upstream from a secondary obstruction in the epididymis) a microscope is necessary because the tubules of the epididymis are so tiny. But one cannot determine before or even during surgery whether a VE will be absolutely necessary, and the relative indications for a VE (thick creamy fluid coming from the lower freshened vas ends on both sides) are evident in only about 5% of patients. For this reason, many surgeons proceed with a VV, which can be performed through a single short (less than 1-inch) midline incision, and which causes no scarring of the epididymis should the patient fail to experience a return of sperm to the semen with a VV and opt for a VE in the future. In addition, a VE requires that the testes be temporarily removed from the scrotum through long (usually 3-inch) incisions on both sides of the scrotum, often extending up into the groin so that the surgeon can mobilize enough vas tube length to fill without tension the longer span between the upper vas end and the epididymis. I think these maneuvers should be performed in a formal operating room under anesthesia. Why impose upon everyone the added expense of an OR and the added scarring of long incisions on both sides when they will prove to be necessary in only 5% of patients?