I think I can answer this for you. I climbed Aconcagua in February (see my TR http://www.summitpost.org/aconcagua-2011-solo-via-the-polaco-guanaco-normalo-route/709034
I remember passing the area you speak of, up at 6250m, at the end of the highest traverse line. There were a couple of tents there, when I passed. Yes there is some flat ground - but a bit exposed to wind I would think. I thought it looked a fairly miserable place to camp - and the two people I met there did seem a bit miserable! (they clearly weren't acclimatised and could hardly put one foot in front of the other).
There is another smaller, but maybe slightly more sheltered, area down at White Rocks at 6070m (see photos in my TR). In both areas you will be melting snow for water, but then you will be at any of the high camps - Berlin, Colera or Camp 2 Polish, which are all at around 5900m. When I was on the mountain the highest I was able to access water from a stream was Camp 1 Polish at 5000m (NB with some work with an ice-axe to make a hole in the ice!). Above that height everything stayed frozen - even at Guanacos 3, at 5500m, where I was told you could normally expect a trickle of water in the afternoon.
Another thought: I was set on the Polish Traverse when I went out. However, in the conditions I had, most were failing who went that way. Main reason was that the ascent from Polish 1 at 5000m to Polish 2 at 5900m is tough with a big altitude gain. Then Polish 2 is very exposed and a bad place to be in bad weather. I changed my plans and did a lower traverse round to Guanacos 3 at 5500m. This is much more sheltered to wait out bad weather - and it was the pleasantest camp I stayed at - lovely view of Cordon de los Penitentes and the Ramada Group. Then it is not hard to get from there to Colera Camp up at 5970m (highest of the 3 high camps).
Like you I considered a higher start for the summit day. But the advantage of another 400m in height is off-set by the exposure of the higher camps and also the discomfort of trying to sleep above 6000m. Colera is fine - a long day - but still quite manageable if you start early, are well acclimatised - and have the right weather window.
When are you going?
Best wishes, Mark