Wait how common are fills? I've only seen one on any of my pipes.
For the first 75 years or so that briar pipes existed they were 100% smooth. The vast majority of pipes made in those days had lots of large fills because imperfections are an unavoidable characteristic of briar, and sandblasting and rustication hadn't been invented yet. Mastic (putty) fills were the only way to deal with the situation.
After blasting and rustication came to the party, smooths became less common---a lower percentage of production. But the ones that did had both fewer and smaller fills.
At the enthusiast/collector level, while most smooths STILL have a few fills, they are usually sand-pit-sized and dealt with using dark(er) finishes and filling cleverness (i.e. non-mastic fill materials), so are difficult to detect.
Smooths with lots of large and obvious fills still exist, they just don't show up often in the racks of guys who frequent boards like this. Functionally, they are "briar corncob" pipes that are used for lawn mowing, camping, fishing, and so forth. Often referred to as "basket pipes". (The label comes from how they are usually sold in B&M's: jumbled in a box as opposed to in a case under glass or attached to the wall with little rubber grabber doo-dads.)