Yes.it is helpful.
Because when we have a book mark with some dimensions,metrics and some filter values in it.(Conditional hide &show)
If one metric name is changed.If we select the same book mark it will give every thing except that changed metric.
now replace option is helpful....because I have all other selections , I can select new metric and replace the existing bookmark.
Otherwise we have to start from scratch.
We can get it in IE plugin.....Can we get it same in AJAX ?