First, arching the back has a positive effect of slightly increasing your reach during the kick, as the displacement of your upper body backwards allows you to thrust your hips forward that small amount. One martial art where you will see this significantly is Capoeira, where the traditional Benção, a front thrust kick, involves arching the back to increase range (as well as moving the upper body away from an attack).
I would suspect that, if you arched your back while chambering a spinning kick that turned the other direction, you could gain a slight bit of extra momentum from moving your upper-torso forward to start the spin, albeit at the cost of telegraphing your movement. Lastly, as noted in this question about butterfly kicks, arching your back can be advantageous because it helps keep your upper torso elevated during the kick.
Unfortunately, it's not all sunshine and roses. I haven't found any material specifically dealing with martial arts, but arching the back while benchpressing has been linked to back injury and diving similarly sees back injuries from arched backs as the diver hits the water. This fellow issues the same warning for a number of exercises involving arched backs, although I take that with a bit of a grain of salt since he seems to be vilifying a rather large number of exercises that people do safely every day. Anyhow, the long and the short of it is that arching your back hyper-extends your spine and puts additional pressure on the discs of your back. It's probably not all that harmful for the kicking itself, but if your motion is interrupted, say by being struck yourself, you're putting yourself at risk of the impact and subsequent whiplash hitting your extended spine and causing more serious injury.
So, in short, you're probably best off not arching your back during the chamber or kick because it could contribute to spinal injury, unless the kick specifically calls for it. As for how to prevent it, your best bet is noticing it when it happens (filming yourself or having a friend watch is probably your best bet. Obviously, you're probably best off not wearing anything baggy for fear of obscuring your error) and then correcting for it. Do the motion as slowly as you need to to be mindful of your movement, then speed it up until you've kicked that habit at full speed.