Why is enabling SSI for .html files a risk?

Hi!
In the security tips on Apache 1.3, 2.0 and 2.2 it is stated as follows:
"Enabling SSI for files with .html or .htm extensions can be dangerous. This is especially true in a shared, or high traffic, server environment."
http://httpd.apache.org/docs/trunk/misc/security_tips.html
Enabling SSI increases server load, but why is it a risk? Does this apply only to enabling SSI for all .html files while the server is very busy (because that could cause a denial of service)?
I'm interested in whether enabling SSI for .html files is a risk for a single virtual hosting account (without/before high traffic).
Thanks in advance, Lars

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