My take on it is a bit two sided. At times it can be useful. For example I wanted more than one Mythoclast. So when I finally got my first one. The first thing I did right away was do another VoG (I remember I still had two more left). What do you know it drops me another one. Or another example is if I'm getting drops I don't want from say CE. I stop and wait a day or usually two and then do another one. This can help change the drops you would of got other than if you had done it really close together. Loot groups? No. Like when you said you got three NLB in two hours. It just likes to give you what you already have got but there are other things to it as well. Like dismantling something. You can get things you have just recently dismantled. Whether or not it is supposed to be like that is beyond me. I doubt it but maybe it is supposed to be made to be that way.