Time, Cost and required skills

The first and the foremost reason for the use of informal research in past is that formal research is time consuming. Most of the researchers are dissatisfied with formal research because a lot of time is needed to do this whereas on the other hand informal research is fast and time consuming. But with the passage of time people and researchers are becoming able to know the importance of formal research as formal research provides us accurate information than informal.
Formal research method surveys are expensive from that of informal research. There is a lot of paper work involved in formal research as it is a primary information. On the other hand informal research consist of secondary information and only a little bit paper work is involved in it.
The person who is doing formal research is required to have various skills like answering on the spot, leadership qualities etc. so that he can do his research in a proper way. Whereas in informal research a person requires less qualities and skills as compared to the person doing formal research.

Reference 
http://www.nri.org/projects/publications/bpg/bpg10.pdf