Hollows and rounds cut 60 degrees of a circle. Snipes bill planes cut nearly 90 degrees, and a different portion.
Much of the time I only use the tip of the plane (red) to inset a quirk, but a full width shaving can certainly be taken (red and green).
Let's look at a few examples of quirks being inset.
A hollow just won't reach in some cases. Additionally, a hollow is designed to cut the circumference of a circle where as a snipes bill is designed to cut both the circumference and the adjoining fillet.
Do you need various sized snipes bills like you need various sized hollows? The answer is probably not, unless your work is very specific. You certainly will not need as many. I have never desired one larger or smaller than the standard size that I offer, which cuts a 5/8" radius circle.
The single size is not at all limiting. The side bead above is 3/4" diameter, below is 1/2" and 1/8".
Why do snipes bill planes come in pairs? These planes do not travel well against the grain. (In fact, many times woodworkers have issues with these planes it is because they're going the wrong way or have chosen wavy grained wood that has no right way.) There are times, as can be seen above, that you'll want the profile the snipes bill creates facing a different direction.
Sometimes the snipes bill plane has nothing to do with the profile. Look again at these two
The profile of the plane has nothing to do with the finished profile. They're used simply to create a void for the hollow to enter. The profile you see is from the hollow, not the snipes bill. A hollow can't reach and cannot cut a fillet.
I know what you're thinking now. "If the profile isn't necessary, just the ability to inset a vertical quirk, What else can it do?" Due to the significant research at Clark & Williams we have been reintroduced to a wonderfully easy way of starting rabbets in square and non-square stock. One of the added extra advantages of snipes bill planes is that they introduce an easy method for starting rabbets in square stock. The plane stays in the gauge line with almost no effort while starting to define the fillet for the rabbet to follow.
I believe Don McConnell of Old Street Tool, Inc. demonstrates starting a rabbet in non-square stock in Traditional Molding Techniques: Cornice Molding. Snipes bill planes excel here.
Using only a rabbet plane in this situation is very difficult because the rabbets are often inset far enough that you can't use your fingers as a fence. Moving fillisters, plow planes and fastening a batten down here will not work.
Is a pair of snipes bill planes necessary? After all the new ones are expensive and the old ones can't be found. They're only necessary if you like what you see above: profiles being next to profiles, quirks, shadows and working with non square stock.
And here's the handle of my nephew's new Harry Potter wand. The story says that he has the feather of a phoenix in his wand...so I'm told.
It's the first time I carved anything in the round. I was pretty excited that I didn't stab myself with a gouge while I was setting it in; the futures in Vegas were trading pretty high.
My middle son, Thaddeus, thinks it looks like his electric toothbrush.