why not? ... because ...
you could be MORE right a little bit later!
LOL
Seriously ...
if you enter with a portion and are right ..
oh well, the market usually gives you another chance to enter with more (pullbacks)
BUT, if you're a little wrong, you can add later at a better price ...
AND, if you're WAY wrong .. you've only lost a small portion.
That's my theory and I'm sticking to it ...
the most important aspect of trading is not how much you make,
but rather, how much you lose!
( or don't lose :)