The issue is that Tom doesn't actually have anything wrong with him; in fact — and I know this is possibly the most controversial thing I've said in this subforum — I'd even go so far as to suggest he's a bit smarter than average. Even the weird shit he does with the alliterative insults towards the idiotic indolent infants and maliciously moronic manchildren online demonstrates an above average vocabulary.
Yes, in practice he acts like his mom decided to take up drinking paint thinner when she was pregnant with him, but he's kind of like DSP in that respect where he isn't technically stupid, he's just a spoiled whiny little bitch who's never learned to think independently.
Dave, on the other hand, clearly isn't mentally well. He's obviously got some kind of schizotypy going on if not full blown schizophrenia, and thus his posts range from a little unhinged to total word salad depending on the day. You can't really communicate with him or attempt to move him in any direction, at best you can agitate him until he goes into a full blown meltdown and runs away.
He's great as a side character, but he's too detached from reality to tolerate as a primary figure on any long term basis.