While a subset of users have true 1080i televisions, most people actually have LCD, plasma, or projecion TVs that are some form of 720p (or 768p). The future of course is 1080p.
The reason 1080i is so common is it is easier to transmit, and nobody saw the price of LCD and plasma dropping so fast, nor the availability of DLP tabletop tvs either. Broadcasters thought the CRT was going to be the main HDTV mechanism for a long while.
But converting to 720p doesn't save that much bandwidth, only about 8%. It is a better format for films and filmed television, but interlaced is better for live events and NTSC taped formats.
Since my TV is 480p, either is fine for me...