My bike came with them, but they were one of the first things that went when I started customising it up ...
I just found I never used them. I like riding on the hoods rather than on the top of the bars and found that I have all of the control and "balance" that I needed in that position.
About the only time that they had any use was commuting - but that only brought the total number of times I used them to somewhere in single figures within a month. I would agree with David about them being more comfortable, but I don't believe that there is any additional safety element - riding on the tops on the flat with your hands close to the stem (where the levers sit) takes away a fair amount of manoeuvrability.
I wouldn't advocate riding with them in a bunch - to my mind, they don't give the braking modulation and force that the "main levers" do. This may be just an impression rather than reality, but as they say "perception is everything ..."