Is there any reason I should not use monospaced typefaces for text in articles? Do they negatively affect the reader's ability to easily read long-form text?
Monospaced typefaces do reduce legibility, albeit by a margin.
In Universal Principles of Design, the entry on legibility states:
One famous research on this is Beldie I. P., Pastoor S. & Schwarz E, 1983, “Fixed versus variable letter width for televised text”, Human Factors, 25, pp.273-277, where part of the results include:
It has to be said that latter research has shown more marginal differences, and that in some cases (dyslexia or programming code, for example) monospaced typefaces increase readability.