Too often I hear of people not sticking to their training or diet, or even both, which leads them to get erratic and less than ideal results. Seeing progress is key to staying motivated towards your goals.
Only by staying devoted to both your training and your diet, will you see the results you want and should be getting. When you hold back and put half the effort into your training and diet then you will only gain half the results.
I hear the saying its '70% diet and 30% training', and use these words myself, so although I agree with it up to a certain point, it can create a mind set that is not always about aiming for 100% in both. I don't know anyone who has a great physique that only puts in a bit of effort here and a little somewhere else. To gain the best possible RESULTS it's all or nothing.
It's not about 1 good day or even 5 good days but about making it into a healthy lifestyle. It can't feel like a chore and must be made into something you just do. The gym isn't something you can do for a while, gain some results and then just stop, it's about being CONSISTENT!
It all comes down to what you are trying to achieve, and how much you want it. If you want the best results then you have to put the work in EVERYDAY.