Is Business Assessment done by External Credit Rating Agencies really helpful for analysis of Credit proposals ?

YES it is helpfull since they gathers account and various other  information from various creditors, Business,  and Government institutions, with link to other credit rating agencies accross the Globe, Since their data has been updated reqularly hence they will have proper understanding about customer financal activitis and its potentials,

The assesment report is also helpful in taking internal credit approal and in case of claiming insurance against defaulter. 

Credit rating agencies assess the portability of default on the short-term and the long-term of a borrower mostly based on the borrower's historical financial performance and other qualitative metrics that normally contribute to maximum 40% of the overall credit rating. So the answer is yes, but partially. Other things should be taken into consideration in the credit proposal, such as: 1) the purpose of the borrowing; 2) the source of repayment (primary and secondary); 3) the viability of the "project" or investment the borrower is contemplating; 4) the borrower's credit history, performance track record and ability to carry out a specific project/investment. This contributes to have a successfully drafted credit/business-case proposal. Also the nature of collateral that is offered (this is a topic of its own) should be assessed, and the underlying financing structure of the project/investment that sometimes need to be ring-fenced against the borrower's overall financial situation should the lender wish to isolate the financing risk from the borrower's overall risk.