Dear Ashish Jain,
I think that most probably the periodicity of payment ( weekly in your case) gives rise to your proposal of engagement of casual labour in works of regular nature like production and the questions associated with casual labour.
Literary meanings of the word "casual" are temporary, occasional, irregular etc. When it qualifies the word " labour", it takes in its mantle the nature of the work as well as the periodicity of the engagement of the worker. In short, when a workman is engaged on an occasional job intermittantly, he is called a casual labour. So, mode or periodicity of payment of wages is not a determinative factor for defining casual labour.
The negative aspects of casual labour being flexibility of hiring and firing, denial of the benefits of regular employment such as occupational wages, leave benefits , security of job, terminal benefits etc turn them out to be a new social class called " Precariats". So, engagement of casual labour in regular works of the establishment is an unfair labour practice and hence illegal.