That is some awful logic right there.
Let's have a look at legs vs. boots. First let's have a look at stats in comparison to a 905 piece for each slot. For the sake of simplicity we'll assume a character with ~50000 int (around 910-915 itemlvl) and look at the 2nd stats as if they were all versatility, for the sake of a rough estimation.
Compared to a 905 item the legs give you 877 int and 894 2nd stats.
The boots give you 657 int and 177 2nd stats.
So a net gain from the legs vs. boots of 220 int and 717 2nd stats. In versatility (So the worst stat, in fact you will gain better stats) this would thus translate to 0.44% from the int and 1.51% from the 2nd stats. That means without even taking the effect into account AT ALL, the legs are already leading by 1.95% in healing.
A normal healing rain uptime would be something between 50-65%, maximum 70%. But let's say you have it down when it counts, so let's go with 80% uptime during the healing in order to offset the fact that more of your healing is done in phases where hr is down (as you have it down when it counts). Your hr itself should then be doing around 10-15% of your healing(which does not get buffed by the boots). Let's go with 12.5. This would then mean the boots would push your healing by (0.8*0.875)*0.1*peopleinside/20. Let's say it is a stackable fight, so on average 18 out of 20 people are inside. That gives us a healing increase of 5.6% in a fringe healing scenario which totally caters to healing rain. Basically the more or less best scenario possible.
Quite honestly if your raid manages to have 18 people standing in your hr at all times then i really envy you. On most boss fights our raid would maybe manage like 10-12.
If we substract the 1.95% healing from the stats, the legs need to do 3.65% of your healing in order to be better.
Edit: The 3.65% I had before are not correct since the logs show healing including legendaries. Basically, if the base healing is x, then boots do 1.056*x= new overall healing. The logs would thus need to show (1-y)*1.056=1.0195 -> y=0.0346 or 3.46% healing in order for the legs to be better.
This means I just need to show you a log which shows the legs at 3.46% of overall shaman healing. (And I'm omitting cloudburst healing in this calculation)
So let's go with the current elisande toplog for a fight with nealry always incoming damag pattern
https://www.warcraftlogs.com/reports...ing&source=101
Or the current #4 skorpyron log
https://www.warcraftlogs.com/reports...aling&source=6
As for a fight with less consistent incoming damage, there are even top trilliax logs which go above 3.45% healing:
https://www.warcraftlogs.com/reports...ing&source=101
Now your argument remains, that this is "useless" healing or overhealing. The thing however is that if you need the healing (aka you are not at 100%), then it is not wasted and does not need to be healed by other means. If there is persistant or semi-persistant damage, so basically any scenario where you are healing 100% of the time, it is flat out useful healing.
The only scenario in which this would be the case would be extremely short burst windows followed by a really long time to heal up again.
The thing is however, that the trinket, jonat's, prydaz and the gloves perform better in those scenarios without the need of 18 people being able to stand in your hr. The legs also give you a retarded 120k hps (assuming 4 mil life) on yourself during those short phases. That means we are always talking about burst windows where you have >2.14mill hps.
Have I even mentioned that the leg effect(not the part you get from stats) does not cost any mana while maintaining the boot effect does? Or the fact that that if you take earthen shield, the effect of the boots drops yet again? Or the fact that the leg effect itself(not the part you get from stats) scales only with your health and incoming damage, allowing a not as good as those logs above player to gain an even greater edge when using the legs?
Yes, there are some boss fights where the legs are certainly not the best. But the scenario where boots are best is a very very specif one, not present in any nighthold encounters