Because it is 'extensible' - they say so in so many words.

"While we really like Opera 9, its many features are somewhat esoteric for most people who simply want a good, secure browser. For that we recommend Firefox 1.5. But for the technically adventurous, Opera 9 will shine." Interesting review alright

Should FF Fanboys be allowed to review Opera?

Why does preferring 1 Browser over another make you a fanboy?