Clay tempering, also known as differential hardening, can be considered beneficial and desirable for certain types of blades, including Japanese swords like katanas. Here are some reasons why clay tempering is regarded as advantageous:

1. Enhanced Cutting Performance: Clay tempering creates a blade with a differential hardness, where the edge is hardened while the spine remains softer. This allows for a sharp, durable cutting edge that can maintain its sharpness for longer periods. The hard edge allows for effective cutting, while the softer spine provides resilience and reduces the risk of breakage during impact.
2. Unique Aesthetics: Clay tempering creates a distinct hamon, the wavy line pattern visible along the edge of the blade. This pattern is highly valued for its aesthetic appeal and is often seen as a mark of craftsmanship and authenticity. The hamon adds visual interest and uniqueness to the sword, making it visually appealing to collectors and enthusiasts.
3. Traditional and Cultural Significance: Clay tempering is deeply rooted in the traditional Japanese sword-making techniques and holds historical and cultural significance. It is considered an integral part of the artistry and craftsmanship of Japanese sword making, connecting the blade to a rich heritage and tradition.

However, it's important to note that the effectiveness and quality of clay tempering depend on various factors, including the skill of the swordsmith, the specific steel used, the clay application technique, and the overall heat treatment process. A well-executed clay tempering process can result in a high-quality blade, while improper or inadequate execution can lead to subpar results.

Ultimately, the effectiveness of clay tempering depends on personal preferences and the specific purpose or context in which the blade will be used. While clay tempering is often sought after for its benefits in Japanese sword making, it may not be necessary or appropriate for all types of blades or applications.