Seamlessness isn’t pretty; it’s opaque and obscures the underlying structures of the tool you are making.
A stitch or a seam isn’t ugly; it’s an affordance that exposes the design, construction, and make of what you’ve made in a way that lends itself to learning.
Beauty and uniformity are two entirely independent characteristics. Seamlessness can look ugly and stitches can be pretty.
Good design can only be seamless when it has just one job to do. Add more jobs and seamlessness becomes a hindrance.source
In an article discussing the state of Macbooks and iPads, Baldur Bjarnason discusses the difference between seams and stitches when it comes to design: