Many reputable books write "nèige" instead of "nàge" in the demonstrative sense, for example this grammar book by Yip Po-Ching.
那一个 is indeed something slightly different, when you are faced with a choice and leaving out the noun you are referring to. (as in "Not this one, but
that one") Perhaps "nèige" has had its use broadened to cases like "nèi ge rén".
AllSetLearning is by John Pasden, a respected language trainer who worked for ChinesePod a long time ago. He recommends also using 那个 as a filler in informal conversation, that's all. Personally, I would use it sparingly, like you wouldn't want to use "Umm" a lot.