Server based aggregation is the way to go these days as it minimizes bandwidth on the server as well as on the client side. However, client side aggregation also has its benefits.
First, users don’t have to wait while loading the data as it is already on the client server. The waiting time may not matter for text downloads, but can be significant for bigger and heavier downloads like videos. One answer to the bandwidth problem is to update sites with a ping through websites like weblogs.com using web applications like blogrolling to the client. The client only needs to visit the updated sites and download the information before the user requests it.
However, this solution may not work well if the client is charged per downloaded data, therefore, server based aggregation may be better option in this instance.