i'll also like to show u the importance of having image stabilizer.
Firstly, take a look at this video clip. It shows u how image stabilizer works.
http://www.youtube.com/watch?v=KPdy52mR6Io
So now u hv an idea of how IS works and how it will greatly help u to prevent camera shake (which can seriously ruin your pictures)
I'll just give u 2 examples, and u can think thru it yourself.
Example 1: Cost
Lenses with build in Image Stabilizer cost more than lenses without (common sense)
Taking Tamron 17-50 f2.8 lens as example, as this is a very popular lens among all brands of DSLRs, due to its sharpness at a very affordable price. Canon, Nikon and Sony all can use this lens. Prices reflects the average street price of shops in Singapore.
Tamron 17-50 f2.8 without stabilizer $610
Tamron 17-50 f2.8 with stabilizer $930
Canon and Nikon users who wish to get image stabilizer will need to pay extra $$$ for it. In the above case, they will pay $930. Sony Alpha uses who wishes to get image stabilizer for their lenses pay $610, as we do get image stabilizer regardless of whether the lenses has build in IS or not. Just think, in the years to come, how many lenses u will buy and how much you will spend each time u need to get IS. Sony users will never get this headache.