Sir, the better question will be why build mosque at all? 'Salat' is not 'namaaz' and many discussions in this forum has proven that. The whole purpose of 'mosque' is waisted. Besides, if only 'namaaz' is to be said in the mosques, while it can be said anywhere, then the whole purpose of a building with or without a crescent is, completely useless. What useful purpose does the mosques serve anyway?
The idea of a building where the problems of the community can be discussed and by mutual consent solutions are found based on the principals and ideas presented in the Quran seems more logical. In other words, their should be a community center, a city hall, not a mosque. I do not recall anywhere in the Quran where God ordains us to build mosques and say 'namaaz' in them.