I wouldn't say "should not" but there is a part of the, shall we say 'traditional' organist fraternity that seems to frown upon the idea. The quickest and simplest comeback to that is of course "Well, it was good enough for Bach!" (ie, his Vivaldi transcriptions which now count among the collections of "Bach" organ works).
I'm not dead against the idea of performing orchestral transcriptions on organ but there's a fine line before you start getting into Theatre organ stuff which can grate on the ear sometimes (especially the oom-pah pedal work). I'd like to see more transcriptions that consider the orchestral colours and translate an impression of them to organ registrations. Example; "Oh, it's an oboe solo here so I'll use an oboe stop." or "there are trumpets here so we'll use the trumpet stop." Sometimes, it will make far more sense to examine the dynamics and texture of the orchestra rather than making it too literal.
Many years back, I did an arrangement of Vaughan Williams' Fantasia on a Theme by Thomas Tallis. In fact, I did two, one for organ solo and another for organ duet (which is a little easier). My basic reason for doing it was that I adored the work so much (still do) that I would love to be in a performance of it. This thought is somewhat hampered by the fact I'm not a string player, so why not do an organ arrangement? In doing so, I had to use registrations which have nothing to do with strings or string technique, but are more in keeping with the dynamic constrasts used throughout the work (the first and final chords are a real challenge!).
Take the sound of a nice French Horn ensemble. I find that a well voiced Diapason is a good substitute for this effect rather than using a reed stop.
So, from that POV, the pros are that you get to play pieces that you might otherwise not get a chance to play if you don't play an orchestral instrument. The cons are, sometimes it can just sound twee or kitsch, but that of course is highly subjective.
Matt