I think psychology plays a big role here.. Sure, maybe going 20 tournaments w/o a cash is not very likely, but how well will you play if you go 5 w/o a cash? That would be a quarter of your bankroll gone.
Yes, and this is just another factor that makes someone a decent player. I myself, tend to play the same no matter what my current session results may be, however I have been known to go a little mad at 100 PLO after dropping a few buy ins.
If you are a newer player, or just not that good, then yes you must adjust for this in your bankroll.
But, again, if you are playing MTTS, you only need to cash in 1 of 20 (not minimum cash) to make your money back and way beyond. If we are talking SNG's, well if you are not cashing in 1 of 5 then you need to play something else because its a losing game for you.(technically you should be cashing in 1 of 4 sngs to actually profit)