This is pretty much completely wrong; roach/baneling decimates thors (or thor/marine). Just engage with roaches first, run in banelings directly behind them, surround the thors, and let the banes and their splash do the rest.
Also, it's extremely tough to spread out thors or avoid banelings with marines. Marine spreading doesn't even become really relevant until mid/high masters (players' micro isn't good enough), and thors are slow as hell and thus very tough to spread out. If they're too spread, you can just pick them off one by one with your roach ball.
Nope, Thors are really easy to keep spread out, especially since most Terran players by mid gold are comfortable with 5 control groups, if I set my marines to one control group, then my Thors and SCVs to the remaining four control groups then spreading is as easy as 'click, 1 click, 2 click, 3 click, 4 click. And if I am going Thors and I see lots of banelings then I just count it as a win and go back to macroing.
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