Thanks cranky for explaining the second reason.
I have been reading about speaker stands and almost everybody seems convinced that stands are a must for bookshelf speakers.
Now, if i understand correctly the 2nd reason : Because 'shelf on the wall' has a firm support on one side (the wall), the other end which is farther from wall(even with angles etc.) won't have that much sturdiness and thus will make the speaker to physically move with vibrations... is that correct ?
Also, one question to all experts is :
what about 'an entertainment centre' (imagine a much longer -parellel to wall- TV stand ) which is :
* an independent (not fixed/attached to wall) sturdy unit with a flat surface long enough to accomodate 2 speakers with proper distance between them
* With the height such that when speakers are kept on it then they are at the right height..
will it serve the purpose or still speaker stands are necessary ? (no rear facing port in speakers)
The only diff. i see then is the more 'vacuum' around the speakers in case of stands, which will be restricted in case of such flat surface of entertainment center (or for that matter any such sturdy furniture unit).
(I agree though aesthetically (also) stands are better

)