That would also help, yes. But a more powerful query system would be a life saver for clients.

Even if you decreased the bandwidth via negentropy significantly, the client still has to process huge amounts of unnecessary data even for very basic tasks. (like displaying a number of followers)

On mobile devices, that can be a big problem as that drains battery needlessly.

Heavy computing should be done on relays. They are built for this kind of work and can implement overall more powerful caching strategies.

It would also help with speed of development as new clients will not have to reinvent the wheel regarding common tasks like getting mentions, followers and other non direct, yet related data.