As Antony said, I am a penetration tester, which tends to drive my research. I have seen several malware based attacks via .htaccess when I worked for a shared hosting company many years ago. The append iframe has always been a malware staple. Although I saw more redirects to specialized exploits based on user-agent rewrite rules than the blackhat SEO ones you are showing. SANS ISC has also covered several of these techniques over the years, but I digress.

The point of my .htaccess based attacks are remote code execution or information disclosure that are valuable to an attacker during a targeted attack. Malware distribution is a very different beast. However, in both cases Apache hardening will help if not mitigate the attacks as dicussed in the earlier comments.