This topic has already been talked about extensively elsewhere, but I wanted to briefly add my own notes-to-self on this blog for my own purposes in listbuilding (at least prior to Codex: Chaos Legions).
Since Greater Daemons require a champion of chaos in order to be able to come on to the board (summoned), the true cost of a greater daemon is not simply the (frankly attractive looking) 100 points, but 100 plus at least 30 (1xCSM, upgraded to champion). In all likelihood, the champion(s) will be equipped with something a bit more effective than harsh language, such as a power fist. Hence the greater daemon might "cost" something more like 155+ points.
In my Death Guard armies, I employ plague champions with power fists regularly (63 points). This really bumps up the points cost of the greater daemon to something more comparable to those found in codex: daemons (but without the cool rules associated with them). But by employing multiple champions, the chance of getting a greater daemon to where its most needed is increased.
When I use summoned greater daemons, I now think of them as being chaos champions that change form at some point during the game. This helps me visualize them a bit better and their capabilities. Doing this for a starting player is hard though -- I used to find myself getting locked-in on thinking of them as champions only and seeing the greater daemon as something separate entirely.