There are pluses and minuses to pistol-mounted lights.
The plus is that the light is always aligned with the muzzle. That can be very convenient and help with rapid, accurate fire.
The minus is that the light is always aligned with the muzzle. Even if you take care to try to light rooms or people obliquely, you're still presenting the weapon toward the person... presentations that may be inappropriate and that you would not have otherwise done if you had a separate light.
Of course, you could carry a separate light for room searches, but that would pretty well negate the weapon mounted light and good identification and sight acquisition can be made fairly quickly even if the separate light is not perfectly aligned.
Currently, rails on pistols are largely seen as the next evolution of the defensive/combat pistol. Of course, recurved trigger guards (as seen on the Beretta 92 and Glock pistols) were seen as the next evolution of the defensive/combat pistol in the early 1980s... and look where that has gone.
I tend toward being pro-pistol mounted light, but I'm mindful of the potential pitfalls. A light is necessary, but I don't consider a pistol-mounted light a "must have" for a defensive/combat pistol.