Trev
It's already been mentioned that the mashout serves 2 purposes, increasing efficiency and denaturing the grain. These are correct as the main 2 reasons, but I thought that I would expand slightly.
As far as efficiency is concerned. The mashout doesn't effect effeciciency of the conversion (well, thats not entirely accurate. But lets say that it is minimal). What it effects (more) is the efficiency of the lauter, and the subsequent sparge. By increasing the liquor ratio, and increasing the temperature, particularly for heavy batches, it changes the amount of sugars that, having already been created and are now in suspension in the water, come away with the water in the first runnings, due to the concentration of sugar/water and the viscosity at the higher temperature. This also means that the sparge has less sugar that it needs to disolve, making the sparge easier. If the liquor ratio in the mash itself is sufficient, this is not an issue anyway. It really is only a problem if the mash itself is too stiff.
Secondly, you have grain denature. This is an issue in large (commercial size) batches, where the grain needs to be denatured before the first runnings, purely because of the amount of time it takes to run out a full batch. In a batch of commercial size, beta activity would continue in the long length of time it takes to drain the tun. In terms of homebrewing, this is not so much of an issue.
So, IMHO, a mashout is not necessary (although with a wheat, of which I have limited knowledge, it may be required because I have heard it can be a bugger to run it out...but that is something an experienced wheat brewer will be able to confirm or deny) if your initial mash ratio was high enough. Having said that it is not necessary, I do a mash out.....not for the purpose of a mashout, per se, but purely that without one, I would not have enough water available to sparge with, purely because my HLT is not big enough
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I was actually running the numbers myself for a double batch the other day, and worked out that I could do it with a mashout, but only just. (I also have a 55L esky). It would have been a tight fit, and I brew low OG.