I'm really trying to find out if I should spend money on a Yun just out of curiosity. If it is really only practical as a Wifi webserver for programs running on the Arduino side I don't think I will bother.
I guess it all depends on what you want to run extra.
You simply cannot compare the memory usage and footprint of you linux with the linino. This because the linino code is written with the size limitation in mind.
Basically you can run any -non gui- program (as long as you do not run out of memory) but it will take longer.
I think the yun excells in mobile aplications. This because you have the wifi and a linux all in one allowing you to have the robot wireless from day one with the confort of arduino as you are used to.
I would prefer a mega yun for robots though as the leonardo is limited in memory and program space and pins. ( @ arduino core team: hint hint hint
For my robot I plan to use a yun and a arduino mega. I'll connect the arduino over usb to the linino. This works perfectly but I still have some minor issues to do the switch).
Note that some people attach a web cam to the linino.
So I think the linino is not only a wifi to a leonardo.
However I don't know where it stops.
Maybe someone can help you out by connecting his yun to the internet and allowing you to have a play with it.