How we are judged depends 90% on how competent and friendly we appear to be. This is where the question of whether people will trust us or not comes into play. However, in the social belief, there are few people who are both nice and competent at the same time.
We most often judge nice people as less competent, and competent people as less nice, cool and (quick addition for those who sighed with relief at this point) the kind that nobody wants to work with. It may take more time to build up the belief that you belong to this small group of friends and are at the same time competent, but the benefits are indisputable.
These impressions determine whether people will trust you, build relationships with you, follow you and support you. It is worth investing in this, even when (and perhaps especially when) kindness is not your strongest character trait.
