so whats the point of going all in then? Big risk for small reward...

Just to let you know, betting about 2/3 of the pot and getting called by flush draws is a GOOD thing!!

Uh? You are not worried that a flush draw calls an all in? I would be delighted if a flush draw calls, as he would be making a HUGE mistake!
Not really a big risk if you've got a good read on the players. Besides, it's pretty common that TPTK is a hand where you don't win large pots...you're looking to take the pot down right there, or get called with Ax or diamond/diamond...hands where you're a significant favorite to win.

I'm not worried about flush draws calling me all in. They call me all the time and most of the time, they miss.

In my experience, the people who call flop all-ins on 4 to a flush are either A)shortstacked or B)weak players who just want action and want to gamble(after all, poker, regardless of the skill involved, is still gambling). And in either circumstance, I'm going to get that money from them one way or another.