I've just counted some statistics on a planet file from 14th of October. Here it is:
This table shows a number of nodes, both tagged and untagged, that are referenced by ways and relations. You can see that nearly 97% of 3.5 billion nodes are untagged, and most of these — 88% — are part of exactly one way or relation. Like, when you trace a building, you add four untagged nodes that are part of that closed way.
98.4% of all nodes are part of something, but only 12% (424 million) have two or more parent objects. This could help with designing a data storage for nodes.
There are equal amount of tagged nodes that are not part of anything, and part of an element. Interesting are these 9 million tagged nodes that are part of two or more ways. The taginfo says there are 2.5 million crossings and 860 thousand traffic signals, so that's a ⅓ of that.
Finally, we have a million of nodes with no tags not being a part of anything. I wonder when someone puts on their OSM saviour cape and a programmer's hat and rids us of these.