updated 1 Oct to attribute the quote correctly
There's been an interesting cartographic discussion of base maps recently in the geo-blogosphere; Stamen Design blogged some notes about their work on Bing maps, which was picked up by 41Latitude which was in turn highlighted by James Fee. The point that comes out from this, as Justin O'Beirne says, is that
"in trying to make a base map that’s optimized for everything, we’re actually creating one that’s optimized for nothing."
Which I heartily agree with. An example is street names, if you are producing a map for city navigation street names are vital but if your map is a thematic one showing a heat map of crimes per square km, street names aren't important and should be removed. 41Latitude suggests having a range of base maps for different uses, this is sensible but of course you can go one step further: The new Google maps API allows you to customise the base map and of course, if you start with a really basic base map (say land and sea) of any service you can always produce a custom base map by adding specific layers (e.g. roads) as you see fit.
How Many Users: What no one so far has mentioned is that how much you customise a base map is really controlled by how much effort you want to put in. In turn, that is usually a factor of how many users you have and how complex your map is.
sketch map courtesy Rosanne Elkins-Bushnell
At one end of the scale, the proverbial 'back of an envelope' map has as its base map the envelope and works fine for a sketch of how to find a coffee shop for one person on a street. Showing a group of people the location of the hotel you're all meeting at works pretty well as a marker on a street map; my maps by google would be fine for this as the data can be displayed as a simple map. However, once you start having multiple layers with markers or polygons you really want to start thinking about improving your base map as screen clutter and usability becomes a real issue, especially if your map service is going to be used by many users and a client is paying you to put it together. You want to choose from a series of base maps to get the optimal base map or even customise it for your particular use.