Would it help if they did?
No (assuming you're referring to an EU military force), but you mentioned security so I was wondering what you were referring to.
My understanding is that currently, EU member-states cannot make trade agreements with other nations and must adhere to the common EU trade policy. So, I'm saying that it would be easier to make trade deals with non-European countries if they could act independently instead of having to rely on the EU to negotiate common trade deals with outside nations.