I'm in the process of building my brutus-type single tier stand. I was wondering how often everyone uses their middle burner, the one for your MLT, and is it necessary? Will firing up the burner have any adverse effects on the mash?
It all depends on your setup. If you plan to mash in a cooler, you don't need it. If you plan to do HERMS, you don't need it. If you plan to do RIMS with a tube, you don't need it. But if you want to direct-fire your mash, you need the middle burner.
As for adverse effects on the mash, I assume you mean scorching. I do direct-fired RIMS and have never had any problems -- but you need a full false bottom and a pump to recirculate the wort.