Here's my 2¢, which is what it's probably worth. I use spinning only, generally with 15# braid. Some years ago I used a baitcaster as well, but stopped because it didn't have the versatility that I wanted.
For many, if not most, types of fishing there is overlap, i.e., the techniques can be used effectively with each, so personal preference is a factor. It is said that skipping lures is easier with spinning and casting accuracy might be better with baitcasting.
One thing that probably most people agree on is that baitcasting is better for heavier lines. Not counting braid, many anglers don't use more than 10-12# line on spinning reels.
Of course, there's a reason you're asking. Do you want to do things that you can't do with your spinning or are you asking because you know many many people use baitcasting and you're wondering if you should too?