Flat vs. clipless is a never ending debate in mountain biking. There are people who are unbelievably passionate about one or the other. Both have benefits and downsides. IMHO, flats have a lot more benefits than clipless, but...I ride clipless most of the time. If you're riding flats correctly from a technical standpoint, there is absolutely no reason to be clipped in (just watch any Danny Macaskill YouTube video for proof that you can do pretty much anything on flats with the right skill set). At the end of the day, I like the feeling of clipless better for trail riding and sometimes that's what matters most --- a feeling. I'll also note, I almost always ride flats when on lift service downhill, during mtb clinics/camps, or when I want to work on technique and not just riding for fun.
Clipless is kind of like skiing a wider ski as a daily driver when you mostly ski hardpack (I do this too). A narrower ski will help you improve faster (so will flats) and you can't use it as a crutch because you lack technique (just like flats). But, when you get into those choppy conditions, want to be a bit lazy and want a crutch, the wide ski is there for you (so are clipless pedals).
For trail riding, I started riding with flats (some cheap pair...don't remember what), switched to Shimano SPDs after a year or two, tried flats again (another non-memorable pedal), back to SPDs. Last year I switched to
Crank Brothers Mallet E and don't see myself changing them again as far as clipless goes. When I started riding lift access downhill with any type of regularity I picked up a pair of
Canfield Crampons and realized "good" flats are pretty amazing. The Crampons live up to the name, the grip is intense and is the closest thing to being clipped in when riding flats (I've tried a number of different flats on my husband's bikes and haven't really liked any of them).
My husband on the other hand tried riding with clipless pedals once, felt like he had to re-learn riding a bike and promptly put the flats back on. To each their own.