Many people feel that halos (aka outline text) around their map labels will help their map labels stand out better and thereby be more legible. However, in many cases the use of halos ends up obscuring the legibility.
Typeface designers spend a lot of time working out the exact proportions of their letter forms and making sure that the “counters,” the blank space in and around a letter form*, are roomy enough and provide just the right amount of style in just the right places.
Unfortunately, halos run counter to the counters! Especially halos that have a high color contrast with the text. In many cases you can provide the necessary legibility by simply altering your typeface choice, capitalization, text color, and size parameters.
Brian Bancroft, @Brian_Bancroft, provides a great example of this in the before and after maps, below.
*Better definition of counter from Webster: an area within the face of a letter wholly or partly enclosed by strokes.