I have heard several of my friends dismiss the idea of a space force as silly and the main stream media in the UK like the BBC and others seemed to portray this as an absurd idea to discredit Donald Trump without actually saying so.
Here is some food for thought:
Humanity has been operating in lunar space (the space between the earth and the moon) for over half a century now. This idea of a space force is not fictional or absurd.
The ability for a military to neutralise satellites from space without launching missiles from earth would be a major tactical advantage, also the ability to launch missiles from space to earth could blindside many early warning systems such as satellites which are watching ground based missile silos.
Having a capable 'space force' would act as a deterrent to any potential aggressor much in the same way that being a nuclear power deters potential aggressors.
The idealistic reason a space force would be necessary would be for it to protect the earth and its people from natural disasters such an asteroid impact and not to be used against other nations - after all we are all human.
final though is on alien encounter, i'm not going to entertain the faux theory sorry joshiebear i know there is a lot of shelved tech out there but come on dude :)
If a sentient alien species did make contact i think one likely outcome would be for them to see the earth as a resource (which dispassionately it is) they would either take it by force, or should a capable 'space force' present itself as an obstacle they might choose the peaceful approach and engage in trade.
Peace out my friend keep making your opinions know.