Who should be a minister?

It seems strange to me that our representatives in legislative assemblies and the national parliament are made ministers even when they are especially not very well aquainted with their portfolios. A lawyer or a judge being made a law minister is perfectly acceptable to me. In the same way, wouldn't you agree with me that a person like Dr. Kalam be made the Science and Technology minister? These portfolios are there for a reason. They signify different dimensions of development in a country. Doesn't it make perfect sense to put an expert incharge of the ministries? Why should an MP or an MLA be given ministerial responsibilities in addition to the constituencies that they represent? Does an MP or an MLA not hold two offices when he also becomes a minister? How will he or she be able to fulfill their duties being in two offices? In my view, the different portfolios must have ministers who are experts in the respective domains. For instance, a retired army general could become the minister of defence, an economist from a university can be made the finance minster and so on. Of course, they can be nominated selected after a careful scrutiny of their credentials by the honourable members of the house. What do you think?