They do have a purpose - they're easy to snipe. They're simple targets that cost a lot, take a long time to return of investment and they have low health. Good candidates to punish your enemy severely with just a few (tech 1) bombers.
You also do not have to click that much, there are in-game options to make capping an extractor a side effect of assisting/upgrading it. If you do not want that side effect, you can also use context based templates. In combination with a recent assembly patch by @Ctrl-K the templates even snap to extractor build sites.