Pete says.... MR's are unfortunately the workhorse of outdoor lighting. The good things I can say is they are small which results in a small fixture, easy to add lenses and they are powerful with many wattages and beamspreads. They do not have a good wide flood when placed within a fixture. They require deep shielding or they are a glare bomb. Deep shielding cuts off the lamp spread which in return makes the lamp spotty (yes I know correction lenses)
As discussed before there is only 1 good PAR lamp which severely limits it's application. I have found that particular lamp to last longer on average than MR's and be more durable. Furthermore it has it's own lens which gets changed with the bulb supposing your not using a lensed fixture.
I wouldn't pigeon hole a contractor who uses this lamp as any less of an installer for using it. That just isn't fair. Using only one lamp on a project may or may not be the right thing to do depending upon that particular project. I do though consider the use of bayonet lamps the sign of an amatuer . Especially those path lights pointed down with no lens (tulips). That lamp has questionable life to begin with and when it's hit with water it seperates from the brass shell and hangs like an orphaned testicle.