'... if my blog is mostly about, say, technology then you can easily see that from the blog name, tag line, or the titles of the top two posts. ...'
One thing in favour of tag-clouds I've found is they are finer grained. If the knowledge domain is 'foo' you can quickly work out how the vocab is skewed to a particular subject. This is useful, unless they are spammed.
'... is it worth the noise it adds to the site? ...'
On the front page, maybe not. I tend to use the maps less regularly than every day. I check the entire vocab, not just the big words (though they do stick out). You're making a fair point though. I had the same beef with the ruby on rails image maps where they take up space & are indecipherable (to me anyway).
One thing in favour of tag-clouds I've found is they are finer grained. If the knowledge domain is 'foo' you can quickly work out how the vocab is skewed to a particular subject. This is useful, unless they are spammed.