Well, fundamentally I agree that what we glean from a practise session depends on our mental state-of-being in those moments. And that, if there is absolutely no focus, very little will be gleaned from practising.
However, rather than avoiding practising under such circumstances, I would prefer to gain more control over my ability to reside within the mental state which allows me to practise effectively. Some of the suggestions given above can be very helpful; finding something to love about whatever it is you are working on (this is capable of changing one's ability to focus, as well as one's energy levels), or change something that enables a new facet of practise to come to light.
At the same time, I think it wise to discern when a 5 minute jont outside will afford one the necessary focus for effective practise, versus staying in the practise room forcing oneself to comply.
If one wishes to do any kind of performing, as danyal is suggesting, practising mental control is important anyway, so why not use it in daily practise to get necessary work done ? One must learn to practise, and practise effectively, regardless of mood. And in most cases, one ends up enjoying it anyway.
If you are playing the piano strictly for a mood-experience, with absolutely no strings attached, then sure, play when you feel like it. Otherwise, training oneself how to practise consistently is indeed necessary imo.
m1469