This question was made over 9 years ago. It made sense then, it doesn\'t make it now. Flash is hard on its way out; sup
I would say the definite pro of using instead of Flash is that I will be able to watch videos in my browser without having it crash or go crazy slow. I use 64bit OpenBSD, so gnash is all I can get and I only enable it when I'm feeling really risky because most of the time I'll come across a flash banner ad and my browser(firefox 3.5) will crash.
And my mobile phone browser doesn't support flash, yet it supports the video tag(and it's not an overly "fancy" phone either).