That is true for the pilot approaching the LS but all he needs to know is what is between him and the LS and if he knows another helo is taking off from where he is planning to land (or very close to it) then that should be his focus.
The alerts differ depending on proximity (or tau in the case of fast movers) so the highest threat is easy to determine.
The departing pilot would note the proximity of the landing traffic and try to identify it before taking off.
No it's not foolproof but a simple system like TAS has saved me on a number of occasions, even in congested VFR airspace.
And no, it isn't a replacement for good lookout but flying with TAS has helped me see other aircraft sooner than I probably would because it directs your lookout into the right area.
And no, it won't protect you from non-squawking traffic but that's not a reason not to have it.