RE prewound bobbins - in addition to the convenience, I think some folks like them due to the amount of thread on them. It's a fine thread, so more than if using a thread that's a bit bulkier. (Not all 50 weight thread is the same thickness - I was recently shocked when using up some old Mettler silk finish now that I'm used to Aurifil.)
To get similar results, you can be sure to use a "fine" thread. If you want something finer than the usual piecing thread, you can buy Bottom Line on spools and wind your own.
Now, for my perhaps crazy question... I've always wound my bobbins by holding the thread that was threaded from the center out straight up, and let it just wear itself off. That's what I was taught way back when in 4-H, did it on my mother's machine, my original Kenmore, on my Bernina, and now on my Janome Platinum that I take to classes. I've never had a problem with any of them... Is this a bad way to wind bobbins? Has anyone ever heard of it being done this way?