Transactional data are marketer-focused and reflect the fleeting present, including information such as the time of day, the specifics surrounding a campaign such as offer and creative, as well as "contextual" information (what the user is currently viewing).
Clearly, such information is both important and predictive as some campaigns will over-perform in certain specific situations (a B2B offering during normal business hours rather than on weekends; or, finance-oriented advertisements on finance-oriented sites) and will underperform in others (say, a business intelligence software ad on a sports news blog site).
That is, transactional data can help inform short term, product-oriented, marketing decisions. But, in the end, transactional data and marketing decisions based upon such data are inherently transitory and unstable, telling us much more about each particular marketing campaign than about individual consumers.
Consumer-level data, by definition, are longer-term and inherently more stable, telling us much more about each individual than about any specific marketing campaign (a car enthusiast remains a car enthusiast both during the week and on weekends, whether they are visiting an automotive-oriented news site or any number of other sites). As such, consumer data are far better suited for driving longer-term value and for being deployed across a wide number and variety of campaigns. And, it is especially applicable to branding efforts which try to emotionally connect to prospects through an understanding of their wants, needs, and aspirations.
Put another way, marketing efforts based upon transactional data actually care very little for the individual. Being marketer focused, transactional-based campaigns try to find the best individuals for each campaign. Consumer data, however, turns marketing on its head; being consumer-focused, consumer-based marketing tries to find the best campaigns for each and every individual. And in the era of Web 2.0 marketing, that's a perspective that cannot so easily be discarded.
So, what's the digital marketer to do? Whenever the data allow, market to the individual, while never losing sight of the specific circumstances surrounding every transaction. Context matters very much, especially when trying to drive short-term results. But, to drive long-term value, target consumers and prospects that look like your very best customers, that are likely to have an affinity for your brand and for what it represents, and that will be the most responsive to your marketing efforts.