Well my reasons were..
2. You have to take decision which part should be kept in server or client.
1. Browser dependency..
for an example: if you type
div1 = document.getElementById('div1');
var div1 = document.getElementById('div1');
//note the starting 'var' keyword
So when you write a big javascirpt work nicely in mozilla and you realize it dont work in IE which dont have a good debugger, It takes a while to realize that the error is due to such a small mistake or a misunderstanding:).