Sure, an easy example is public voting. When congressional votes are public, it is very easy for special interest groups to reward or favor politicians who vote correctly. It's easier to buy votes.
Otoh lots more information available to the public often also makes it easier to detect lying and corruption.
It is a valid concern, but I think as usual it is more about if this data in possibly available, not if there exists such a service as this. Because if it is, then there surely is somebody, who has this information, as thus possesses power that you (and me) do not possess. In that case I'd argue that public availability is somewhat better. In the end, you do know that somebody is buying votes, it is just that you cannot do that. It is very much possible that they already have closed back-office system like this.
It concerns me more that "politician" is a very ambiguous word. Basically everybody active enough is a politician, even though he may or may not be in senate, belong to some party and such. And an open dataset on "pretty much everybody" seems a bit more questionable than on those we consider "being in charge".
I agree that readily available to the average person tends to be better than possibly available
In practice, aggregation services like this tend to have more benign results like enabling the bulk emailing of politicians. Others include the suspicion that the popularity of Theyworkforyou, a UK site run by the same people which parses open data on parliamentary speeches and votes, might have had the effect of encouraging MPs to make brief, inconsequential speeches in order to boost statistics on their relative activity in Parliament. Another area in which the effect of their data aggregation might be considered questionable is where it's used as an authoritative source that a particular MP is "strongly against" a particular "cause", when in actual fact they may simply have voted with the party line against a particular bill related to that cause purely due to concerns about one specific aspect of that bill.
