Flexible is OK if the boat is designed to flex. Flexible can be bad if some parts don't absorb the flexing correctly. You can get fatique, cracking and other problems.

Baidarkas were not built the way they were built so they could flex. They flex because of the way they are built. Since they were flexing anyway, the builders added features so they could flex in a well behaved manner. Little bone pads were probably there as much to protect against abrasion as to make the boats more flexible.

A flexible stitch and glue would have to be very carefully built to keep from self destructing. However, 3mm plywood with some glass is plenty stiff enough for use in non-extreme paddling conditions.

> Don't rush to make the boat stiffer, though. Even if it is a flexible as
> an inflatable with a slow leak, you may find that this is a desirable
> attribute. Baidarkas and many skin/frame kayaks are designed to flex so
> they move easily with the waves, so you don't fight them. A 3mm boat may
> emulate that characteristic better than one built from 4mm. Try paddling
> it for a while. If you like the effect, pat your self on the back. If you
> don't, glue in some stiffeners.