bcos in manier cases things will change in later phases.

You cant plan only for the current phase of the project as 
your future activities are still coarse granular. To have 
good plannig you need to have fine granularity w.r.t the 
tasks to get clear WBS

Every project (be it OO or otherwise) has certain unknown factors. It is just impossible to know every constraint in advance. Many things are discovered during detailed design and implementation stage. Hence too much of planning is a wastage of effort when unknown factors are bound to pop up. Hence the focus should be on flexibility rather than rigidness that comes with excess planning.