I wrote a whole big post here but hit reply just as MN went into "offline for an upgrade" mode - cheers HQ!
I am a HV and I would agree with the statement. The reason for agreeing is that I all too often see people struggling because they have had bad advice.
Some babies are on the 0.4th centile because that is where they are meant to be - getting some HCPs to realise this is a nightmare, although all HVs trained in the last 10 years should have had training in epidemiology and looked at centile charts as part of this.
My biggest problem is ion the first six weeks when babies are still adjusting to whatever line they will follow - some will go from the 9th - 0.4th and then sit on the 0.4th quite happily while various HCPs go collectively crazy trying to get the baby back to the 9th centile 
Poor weight gain is a different kettle of fish and DOES need reviewing. In nine cases out of ten it is down to a feeding problem. If this is the case then sorting out the feeding sorts out the weight gain. In those cases when I pick them iup I start off by making sure it is only me seeing and weighing the baby so you don't get discrepancy with two sets of scales. I find out what the baby is doing with feeding and then advise accordingly. Once the baby is gaining weight again then Mum can go back to clinic for weighing if she wants to - I don't advise it while the weight is an issue though - too much conflicting advice. This works fairly well and I have a good track record in supporting breastfeeding now. Does my self confidence in my ability to support breastfeeding mothers the world of good too.
I always tell student HVs to "look at the baby" and you will often find the answer. Is the baby well otherwise? Has the Mum any concerns?
If the baby is well and happy and just following the 0.4th centile (very occasionally I see them following a line below) then there is not a problem generally.
If the baby is not feeding well then help sort this out
If the Mum thinks her baby is unwell then take her concerns seriously.