For one thing, NICE (the UK NGO responsible for analysing evidence and making recommendations on the efficacy of treatments and their cost effectiveness) and an all-party Parliamentary group found signifcant evidence to recommend Mindfulness as a treatment for anxiety and mild depression along with traditional cognitive-behavioural therapy and SSRIs.
Secondly, mindfulness feels, to me, closely related to CBT, a tried and true therapy. It appears to be very closely related to CBT in what it helps to control: intrusive thoughts. The increased focus on meditation is why it's more effective for anxiety and mild depression.
It's most certainly NOT recommended for serious cases of clinical depression. As often, selected application of evidence and over-generalisation is the problem here.
Mindfulness can help with a range of relatively-minor mental health issues, and it can also help prevent relapses in those that have recovered. For those of us who have been helped that's really not something to diminish.
It's not that there's no evidence for its effectiveness at all. There is.
(Disclosure: Mindfulness helped my anxiety a lot. I believe it to be more than placebo, but then I would, wouldn't I?)