The entertainer career has a comedian track and a musician track where you play instruments but saying that putting your sim in the entertainer career and having them busk with an instrument while having your sim sing, or just sing at a microphone is the same as a dedicated career for singing is something I disagree with. It's a substitute that might help tide some imaginative players over until we hopefully and possibly get an actual career for it in the future, in my opinion.
If your sim is famous for singing but is still only doing gigs that isn't completely realistic. Many (not all of course) singers will pick a label to help promote them or make music videos and so on. They get to perform on stages and not just at lots where sims might pass by and hear them sing. The entertainer career with the singing skill on the side is not a viable substitute for a dedicated career that would actually require the singing skill. I find it pretty ironic that they brought Baby Ariel into the game considering what she does. Which is sing and make music videos.