In reply to needvert:
As above, take a look at any fly first tent and it'll have loads of guylines that have to be used to give it any sort of stability as the fly has no 'floor'. An inner first tent is 'complete' (as in wall and floor) and when you put the poles in it becomes a fairly solid structure without pegging or guy lines.
The often heard argument that they're rubbish to put up in the rain has never been an issue in my experience.(My first inner first tent was a North Face Westwind in about 1986) The inner tent fabrics that are used are so light that the small amount of moisture they may absorb as you pitch them in rain is soon dispersed by body heat as soon as you get in the tent.
I'd take a more roomy, more stable inner first tent than a tent with loads of guylines and a 'saggy' inner tent every time.