ID 90 better on a 'route' ie.. LA to MIA via DFW or NYC as its 10% the fare for the route..ZED are cheaper point to point.. when those points add up.. it gets costly..
Recently bought Vancouver to Cabo on USAIR for $107 usd online... Zed tickets via LA were 120 usd.. TAXES x 2 ect..
Also ID routes generally allowed J and F.. so you could buy premium travel.. Zed = Y