Why do spam emails often contain gibberish?

My belief is that the spammers believe that putting gibberish in emails can get those emails past a larger fraction of spam filters. I expect that does happen, although I don't use a spam filter on my personal email. I can imagine a spam filter marking something as spam for short length, presence of certain keywords, or presence of a number of words together. Adding gibberish would increase the length and dilute the keyword's percentage of the total text.

Possibly the appearance of gibberish is to filter out even slightly sophisticated potential victims. In http://research.microsoft.com/pubs/167719/WhyFromNigeria.pdf Cormac Herley of Microsoft Research shows that spammers want stupid, stupid people as victims and might deliberately include misspellings, gibberish and outlandish claims to let smart potential victims filter themselves out.

I doubt you can discount any possibility, up to and including things like "spammers are so stupid they don't notice the gibberish", or "spammers buy really stupid spamming setups that put gibberish into the emails". Given the prevalence of backdoors in underground products, it's hard to put anything past spammers and those meta-parasites that prey on spammers.