Even in instances in which logically the client might only need one of something, I sometimes still wind up having a desire to avoid limiting the channel to a single entry for one simple reason - if I do that, all the power of EE's publish and expiry date is lost.
When I know for sure that I don't need to provide some of those entry-specific functions, global variables or better yet, low variables, are ideal. But for something like a homepage's welcome message, as another example of which you may figure there's only the need for one - consider that the client may wish to take advantage of the publish and expiry dates to have a new version of that entry self-publish on a given date/time, while expiring the previous entry. A retailer, for example, may have seasonal versions of their homepage welcome.
I know your example referred to contact details - which would certainly be even less likely to change, and when you need the full range of available fieldtypes, channel entries are helpful. The reason I chimed in here is to remind you that there are some circumstances in which there may be some advantage, evaluated on a case by case basis, in avoiding limiting the entry to a single item. So an add-on like single entry is excellent for those times when you need the power of channel entries but you can saw with certainty that some of the publishing features that come with a channel entry (like publish and expiry) are not needed.