Previous very long thread on the topic.
I feel pretty strongly about this issue. Relatively. When I became an editor of my academic journal, I wrote a four-page memo to the Editor in Chief, recommending we change from two spaces (“French spacing”) to one space (“standard spacing”). Everyone laughed at me, but we did make the switch.
In a nutshell, the idea of double-spacing is left over from a generation of high school teachers who learned to type on manual typewriters. A manual typewriter, being monospaced, is just about the only place where double-spacing is appropriate. But since these people taught us to type on computers, they required us to use double-spacing in all typing, even with modern proportional fonts. Double-spacing increases the incidence of unsightly rivers, and contrary to the perception of some, it actually decreases readability.[sup]1[/sup] Every major style guide eschews double-spacing, including GPO,[sup]2[/sup] APA,[sup]3[/sup] and Chicago.[sup]4[/sup] Unless you’re using courier new, just don’t do it.
[sup]1[/sup] See Colin Wheildon, Communicating or Just Making Pretty Shapes - A Study of the Validity (or Otherwise) of Some Elements of Typographic Design (2005).
[sup]2[/sup] ¶ 2.49
[sup]3[/sup] Available at [noparse]http://nutsandbolts.washcoll.edu/apa.html[/noparse].
[sup]4[/sup]§ 6.11 (“Space between sentences in typeset matter, one space, not two (in other words, a regular word space), follows any mark of punctuation that ends a sentence, whether a period, a colon, a question mark, an exclamation point, or closing quotation marks.”)