In Flex, styles are used to define the style properties of objects. If you want to write a component where the layout and positions were defined by styles, you could. On the other hand, if you want to completely abandon the use of CSS and styles, you can also write a component that did this (although hardly without using x.setStyle (...) calls for library components).
Saying this, they are as useful as you want to create them. I can give some examples when they will be useful:
. (, ..) , , , . , , ?
. , 5 10 "setStyle" AS3 , mxml , "stylename" , css,
, , , . , Flex, ..
, CSS ( ), . , . c'est la vie. , .