The major advantage of collecting descriptive research data is that these data can provide you with evidence to prove or disprove hypotheses that you might have formulated about your marketing. The main disadvantages are that the data can be expensive and time consuming to collect and are only good for specific purposes.
To help understand this, let us remember what descriptive research is. This is research that is done to help answer a specific hypothesis. Marketers do exploratory research to help them develop hypotheses about things that are important to them. They then do descriptive research to determine whether their hypotheses are accurate.
Therefore, the main advantage of collecting such data is that you get data that helps you answer questions about your marketing. This allows you to, presumably, do a better job of marketing your product. This makes descriptive research data very important.
However, this data does not come for free. Conducting the studies needed to collect it typically costs time and money. The data are then typically not very useful for things other than testing the specific hypothesis for which they were collected.