IIRC, if you import seaborn, then pandas plots will use seaborn styling. I don't think seaborn is a standalone plotting library - it's merely provides styling. So you can have your cake and eat it too.
Seaborn adds new plotting functions, not merely a new style, though I'm not sure that qualifies it to be called standalone. Certainly Seaborn has matplotlib as a dependency.
It is wrong to say Seaborn only provides styling. There are many types of plots available in Seaborn which are Not available in Matplotlib. It might not be extensive, but what it does, it does better than Matplotlib.