Because rating systems are closed, i.e. they have no connection with each other and because they have different parameters and even different rating formulae, comparisons between them are useless. Within each system all one can do is compare one's rating to the other players within it. The actual number of one's rating is unimportant.
Socksey has "never been as low in any other system". This doesn't mean that the system is bad, only that socksey has had a long bad run in FLG and is close to or at the lowest point in her rating range. In a year's time she may well be 100 rating points higher than she is now, but I am not sure that learning from vic and resh is the way to go!

"Useless" is too strong. For an individual it's difficult to make sense of two disparate ratings but for a set of players you can expect a correlation even if not a n accurate mapping. High in one system tends to be high in another, likewise with the lows. DailyGammon is comparable to Fibs, to a degree and even BrainKing's whacky rating system (modified Chess formula, unequal amounts for winner and loser, no match lengths considered) is still comparable to DailyGammon's. We've been discussing the former only just recently, in fact. http://www.dailygammon.com/bg/forum2/main/read/25784