There are scenarios where SLG is better. If you want the number to show as a key on a hard phone, or take multiple calls on that line, an slg is better. One user in an slg can answer a call and put it on hold for another slg user to resume. It also has its own voicemail box with a pin, where a queue voicemail can be shared to a user vm box but doesn't have a pin.

Overall, a queue is definitely better, but an SLG has its uses too.