If Google isn't going to support (i.e. provide security fixes) for the branch that supports older versions of OS X, it would be irresponsible to let people continue to install it.
Safari 4.1.3 is the final version for Tiger as of 18 November 2010.[9] Despite not having received security updates since then, Tiger remains popular with Power Mac users...
Plus new HTML features, but they are secondary.
But, half-joking aside, it's dangerous to run old browsers with no security updates. It might not have bitten you, and it might never bit you (like how you can run off a red light lots of times and you might not crash into anyone).
That doesn't make it less dangerous than running a browser WITH security updates.