Assume for a moment that it's harmful if the data, including IP addresses, timestamps, unique IDs, etc, gets shared with the world. The data was previously inaccessible due to technology, now it's only limited by the policies of the holding company. Some people don't trust those policies (or the comany's security) as much as they did the old model.
So is it harmful? The timestamp / IP combos place you at a given place (most likely home) for a period of time. There are dozens of other companies with the same data, so unless you're being super cautious, it's not worthwhile to worry about Steam especially. If such data were searchable online though, one could have some problems with robberies and with lying about ones whereabouts. The fact that they count "playing a game", not just being on steam, doesn't help with accuracy -- some people just leave their game open while doing other things. The chat histories are similar to what any other chat company does.
Steam *could* do some nefarious things with the gaming-specific data. For example, they could sell it to employers and others who may be interested. The information isn't enough to ruin a life though, and if they use it for anything more evil than advertising they would lose all consumer trust