I have battery and anyone who already has battery would not want just another drum sampler. Instead, I kinda want a sampler and instrument rack integrated with sonar. I think Instrument rack could be a drum sampler. so it might be the same idea.
imagine If you right click on a wave clip and chose something like "slice it to Instrument rack" then sonar create a new track and now you can hit midi to play sliced samples. and also you can edit each samples, like changing the start point, pitch, level, etc.
if it can be replaced by 3rd party vsts like battery, then we don't really need it, but I think built-in samplers could be nicer sometimes because there would be less steps (maybe just with a shortcut key)to work on.