I think it should be taken seriously, in the sense that there is a significant chance that it is true. I agree that Less Wrong in general tends to be excessively skeptical of the possibility, probably due to an excessive skepticism-of-weird-things in general, and possibly due to an implicit association with religion.

However:

1) It may just be false because the big world scenarios may fail to be true. 2) It may be false because the big world scenarios fail to be true in the way required; for example, I don't think anyone really knows which possibilities are actually implied by the MWI interpretation of quantum mechanics. 3) It may be false because "consciousness just doesn't work that way." While you can argue that this isn't possible or meaningful, it is an argument, not an empirical observation, and you may be wrong. 4) If it's true, it is probably true in an uncontrollable way, so that basically you are going to have no say in what happens to you after other observers see you die, and in whether it is good or bad (and an argument can be made that it would probably be bad). This makes the question of whether it is true or not much less relevant to our current lives, since our actions cannot affect it. 5) There might be a principle of caution (being used by Less Wrong people). One is inclined to exaggerate the probability of very bad things, in order to be sure to avoid them. So if final death is very bad, people will be inclined to exaggerate the probability that ordinary death is final.