Saying a Haiku is 'just a warm up' is what people do when they don't understand them and can't write good ones themselves. That would be like saying a Sonnet isn't 'real writing' either, because it's almost identical - the structure and content define the format, and the inherent limitations are what produce such quality results. You can't just count out syllables and call it a Haiku, traditionally that's the least important part of the three main criteria - being 17 'on'(japanese character-syllables) long, divided into three chunks; a "cut" word used to show a shift or change in the poem, and a seasonal word to set the tone - of which, in Japan there are books that are just huge collected lists of categorized, haiku-appropriate seasonal words.
The complexity of the format doesn't allow for too much content, but that's the point; it's subtle, yet nonetheless deep and meaningful. I think a great deal of Poetry these days is lost on people who are too stuck up, and rate poems on objective scales based on instant-understanding and structure, rather than subjective content like emotion or meaning. Haiku is a challenge, because too many people waste words; a master can fit hundreds of concepts, feelings,and thoughts into a few brief words with haiku, while many 'real writers' can hardly express one idea soundly with entire pages.