Google changes algorithm at least once a day:

at least once/day.

crikey - I didn't realise it was that often.

Recently I've been using some search result tracking software that shows a graph of search result placements in time - you can see placements go up and down, presumably due to changes...its geeky but interesting to play with.

Yes they always do that,in order to provide some quality information to the users

Due to number of spamming to increase SERP ranking many one do a lots of spamming or you could say Blach hat SEO. So google time to time keep changing it's alorithm so that it's ranking been not be a well patronized it's should be show a good and quality results.

Google has to keep adept of those trying to get a leg up, so it has to change its algorithm to thwart others everytime

Google does keep on changing its algorithm because it always wants to show more precise results, they probably make 1 or 2 extremely minor tweaks each week.

Yes it is true. Google daily updates the site to stop the spammers.. this helps to maintain the Quality link building in the site.

To produce quality results Google made changes into its algorithms and it helps some businesses to get reputed image.

I don't think they change their algorithm every day. They change it when it is necessary or they feel they would need to encounter some new challenges. They have recently change their algorithm because the new CEO is more determined to rank the quality web content websites.