He could discuss the less crtitical parts of his work. If there is a meetup near his home, he could go there and try to find someone to cooperate with. Or if he is expert at genetics but less expert on math, he could ask someone to help him with statistics.

Suppose you were a professional researcher looking for statistical help. Would you (A) go to a LessWrong meetup, (B), give a talk at the Statistics department of your hypothetical university, or (C) ask your colleagues which statisticians or statistically-literate graduate students they have collaborated with recently?

I'm sure the LessWrong community believes in statistics, which is good. But I don't believe the average member of this crowd is any better at the humdrum practicalities of statistical hypothesis testing than your average working scientist. I would guess LessWrong skews younger and less expert.

Also, he could just spend here his free time, if he prefers company of rational people and has problem finding it outside of his work.

You will not have a hard time finding smart rational people on the Harvard campus! Or, for that matter, near any major university.

I'm with twolier -- LessWrong is fun, but I don't see it being all that professionally valuable for people in most technical fields.