Neither do much - and nor does anything else that's not ridiculously excessive - versus an organised small group (i.e. one which can speed build in parallel from a system or two back in the chain), once the length of the chain becomes significant. They're even less effective versus an organised small group if you think that the "prevent help" option does anything productive, and therefore don't even consider that you might have been in a race at all until you log in one day to find your ultimate target has already been claimed by the parallel build.
It's far better for Frontier to not provide any protection against this sort of thing (and therefore make clear that it's all first-come first-served and you don't have any automatic rights to reserve future systems, unpopular as it might be with some), than it is for them to provide ineffective protection and therefore get all sorts of complaints when people bypass it.
A lot of these ideas would also get in the way for group projects where you don't want (or at least, have no requirement for) a single architect to own the entire chain. I don't know how many different architects there were e.g. on the DW3 chain, or Canonn's chain to Polaris, or the Mikunn one still heading for Lagoon, but the point of a lot of those big projects is that you can just haul to them, start the next link in the chain, and then the next people along finish that link and start the next one, and so on.