Boss, look at this way: There are no Muslims or Hindus or Jews or Christians in the world, there are only people (quite few billions of them) in the world with varying degrees of faith/belief in the idealogy of their choice or the one they were handed over by their respective parents/culture/society.
Does that make it any important to view the religion as a final defining way of life for them, deviation from which will classify them any less moral humans? No, of course not. What's more important is that how much they identify themselves with a certain faith regardless of how much they actually abide by its principles in their daily lives.
Identifying oneself with a certain group is also one of the needs that we have as social animals. Whether it's ethnic or religious or cultural or whatever else. When we say we are Pakistanis, we are identifying ourselves with a particular group. Even if somebody says he/she is an athiest, he/she is still identifying himself/herself with a certain group. So it's quite natural for anybody to have such a group association. We all seek identity, one way or the other.
Now comes the question why do we like to be Muslims or Hindus or Christians. Well, it is my guess that we basically require a certain level of spirituality to take our minds off the ever inflicting questions regarding life after death, or meaning of life etc etc (you know, all that good stuff).
By following a particular religion, we somehow achieve that certain level of spirituality or feeling that our asses are covered once we die because we followed certain faith which promises a better life after death (and we might even get to get a lot more sex than we ever dreamed of in our worldly lives - why do you think all this Taliban dudes are so eager to achieve "shahadat"? Yep, everything in life, death, and life later on is about sex).
But people follow their religions or faith only to the point where it meets their spiritual needs and make them feel good about themselves. For some people, five times a day prayers, fasting, and Haj is the limit, for some, a lot less.
But is it justifiable if somebody practices his/her faith lesser but still be part of that group identity? Well, I don't think it's important. Whatever works for him/her is all good. We do pick and choose in life. We pick and choose in every single aspect of life. Our lives, life styles, brand of six pack in the fridge, all are reflective of our choices - religion is no exception.