To be honest, Rain Teams aren’t the biggest things in the meta right now.
EDIT: To be honest, I feel like rain could be really good. Kingdra will outspeed Cinderace (watch out for whimsicott tailwind), although I personally would never lead Cinderace into A Kingdra Team, and hurricane will hit Rillaboom and is 101% accurate in rain. There are some common weaknesses in the meta right now, mentioned below.
Lightning Rod is probably unnecessary due to bad electric types like you said, but I’ll try and give a full analysis.
1. Rain Teams in VGC 2020
When the Isle of Armor was released, there were quite a few Politoed+Kingdra Teams. This is still decently popular (Politoed is #29 in usage on Pikalytics ) and really the only rain-focused core right now. This team doesn’t really benefit from lightning rod because Kingdra isn’t even weak to electric type attacks.
Looking at other water types in the meta, the most popular ones are lapras and Gyarados, neither of which really need Politoed as Lapras almost always Dynamaxes and Gyarados doesn’t need rain to sweep. These would appreciate lightning rod more, especially Gyarados. I wouldn’t sah that most people are building teams around Gyarados right now (it’s mostly just a generally good sweeper/max candidate) but people definitely are building teams around lapras (including myself). Usually you’d do something with trick room/tailwind + weakness policy/helping hand/fake tears.
2. Threats to Rain Teams
The standard Rain Team of Politoed + Kingdra isn’t really threatened by electric types, they’d hurt Kingdra but not as much as other rain abusers. The same goes for Rillaboom. Really only Politoed is threatened, and it is mostly used to set rain which usually doesn’t need to be reset due to the fast pace of VGC, so it’s not the biggest deal if it goes down. Kingdra’s main threats are other weather Setters, faster Pokémon +Tailwind, and Togekiss.
EDIT: I forgot to mention, Rain Teams don’t like other weathers at all. Tyranitar and Torkoal are both slower than Politoed (And Pelipper), so they don’t even have to waste a turn switching in. Kingdra really needs rain to shine and it’s not as easy to set.
On to Lapras. Unlike Kingdra, it is hurt by electric/grass types, so redirection could actually be useful. The Team I use has Bouffalant which is very nice to bait a grassy glide / venusaur sleep powder/vine lash. I personally find sap sipper to be better because, again, electric types are uncommon, but against Pokémon like Porygon Z, Magnezone, and even Thunder from opposing lapras, Lightning Rod Could be nice. Note that Duraludon often carry thunderbolt, but Lightning Rod doesn’t work with stalwart.
EDIT: 3. Best Rain Setter
As to why Politoed is used over Pelipper, it’s because it’s much better as a supportive mon. Dynamax Kingdra with life orb would probably have very similar calcs, so you don’t need another Pokémon with the same coverage. Politoed, however, can helping hand, icy wind, perish song, and hypnosis (not recommended) whereas Pelipper only gets tailwind, which is usually unnecessary, and icy wind.All in all, Kingdra can hit everything that Pelipper can, possibly harder because of Helping Hand Politoed.
4. Best Lightning Rod Users
Based on the the threats to rain teams, I think that Alolan Marowak
would be fantastic on a lapras team, especially in trick room. It will redirect electric attacks, can hit Magnezone and Rillaboom super effectively, and is immune to both max lighting and max strike from Porygon Z. (Of course, you have to watch out for dark pulse.) It’s low speed would make it amazing on a trick room lapras team, and although you don’t usually play against electric types, the surprise could give you an advantage to win a Best of One. You’d just have to be very aware of whether or not to set the rain in case of marowak’s flare blitz. I can see raichu + gyarados, as it offers fake out, nuzzle, helping hand, and maybe Manectric for snarl, but I don’t think it’s as good supportively as Raichu.
The common Rain Team of Kingdra and Politoed doesn’t really need lightning rod at all, and you don’t really play against electric types that often. However, it can definitely work with other semi-rain teams (Basically, a water type max candidate), especially raichu’s good support Movepool and Aloloan marowaks’s weakness coverage for lapras plus also working well in trick room. Sap sipper is arguably better because of all the Rillaboom, though, but it doesn’t redirect nor have the best users.
Hope I helped!