In Tokyo I've have the privilege of attending several tea ceremonies, a very long process emphasising polishing the inner spirit through outer precision and practice. The practitioners I've spoken to regard it as in the same category as aikido - a budo, more so the more advanced their skills seem to become. In the ceremony their fingers are always together, for the following reasons, as I understand it:
* The smooth straight line created emphasises the earnest direction of intention,
* The straight arm is in direct connection with the hand through a shared line. This is a philosophical ideal to aim for, as well as an aesthetic one because of the elegant simplicity. Picture the parallel lines of the interlocking tatami, and the harmony of peaceful elegance of the sparse Japanes-style room.
* It is, of course, easier to maintain strength and stamina with the support given by the rest of the hand. There is a formal process for stoking a firebox; most of the utensils and containers are made of cast iron, and the straight wrist and straight fingers are important to be able to do this. Further strength is also utilised by the directed extension, which uses additional muscles interior to the arm and can absorb more force than otherwise.
* Broken fingers hurt.