This is a VERY good question, but I think the Keyword here is 'REALLY'.
As for 'need', this can be subjective.
(.. long grandfather story coming..)
Way back ago when I still was terrified of Flash Photography, I felt I don't REALLY need one - those intimidating looking largish hotshoe flash guns. But, I still NEED to have that little built-in puny pop-up or pull-up microbic flash. But then as the years went by and I begun to grasp the techniques of Flash Photography, I regarded the puny built-in flash with disdain - however, it still cannot be totally discounted from use in adhoc shots.
With more finesses garnered through practice, reading, watching, and lecturing from Masters, the Dedicated Flash or Strobe, became an INDISPENSIBLE 'need' to me, but I rarely or ever at all used one without a diffuser - DIY or Gary Fong-ish contraptions. Recently I actually forked out S$88 for a Gary Fong LightSphere Universal - and my goodness why I wasn't surprised; it provided similar lighting quality to my DIY food containers I slap on habitually. Only difference is, this one doesn't pop off or drop and clatter embarrassingly onto the floor in front of all my subjects. :embrass:
So therefore, in my opinion a Dedicated Flash/Strobe is good, using it direct is BAD. :nono:
But going forwards in Photography without a Dedicated Flash/Strobe (either by choice or economic reasons), can potentially stunt your growth in this Art Form - unless you have decided to be a Dedicated Night or Ambient/Low Light Photographer, this can be an exception then.
