The thing is that each change in texture means a state change for OpenGL. State changes are expensive, so you should try to avoid them anyway but in addition, polygons with a different state (i.e. texture in this case) can't be rendered in the same draw call. And draw calls are the most expensive things in OpenGL.
jPCT tries, both in compiled and hybrid mode, to minimize these by batching geometry based on the textures used, so it should be ok if a terrain uses 1-8 texture depending on the size of the terrain. But it's not a good idea to make it use 64 textures or something. As a rule of thumb, I suggest to merge textures where possible but don't go crazy on it. If it really helps your project or your art pipeline to leave some textures as individual, then do it.