as Wordpress 2.9 comes with built in post thumbnail features, is it necessary to use Timthumb ?
With post thumbnails don't have the ability to dynamically crop and zoom images.
Another solution is to use Justin Tadlock's Get The Image Plugin. The Plugin tries to use
the_post_thumbnail() wherever possible and gives you full control of sizing cropping and displaying images. It also allows you to grab images by custom fields.
It is not necessary, but many WordPress themes and plug-ins still use it.
If you don't use any of those, then you don't have to have/install/use TimThumb. If you do, then you are likely to have issues without it because most of those themes and plug-ins are overriding the default functionality deliberately, or are not passing along images in a way that WordPress' default thumbnail feature knows about it.