I guess I don't really understand why Nintendo even programmed the rain to fall so frequently. Some animals (or more accurately "materials" to turn into elixirs) are more common and merchants apparently sell different things in the rain. I still don't think that balances out.
It's the biggest FU from the game designers, and shows a total lack of respect for the player's time. I hate when games do that. I was on my way to a shrine when it just started to spit rain. Didn't even see much in the way of clouds that would signal a rain shower. I was basically stuck in place until the shower passed. So I just put my controller down and started answering text messages. BotW is extremely lucky that it has built up so much goodwill over the time I've played it. I've dropped lesser games for trying to pull **** like that.
You can make five movements before slipping, and I've found four climbs and one lunge cover distance just fine in rain.
Except you're wasting way more stamina doing that than just climbing normally. You're covering less distance and using more stamina. It doesn't add up.
I understand your opinion of the rain, but I don't share it. Some may see it as an unnecessary nuisance, but I find it to be an important aspect of the game. If they were to make it easier, I think that would defeat the purpose of the rain. The rain is there for multiple purposes, of course, and one of those is building a narrative about the reality of the game's world. This is where BotW does it possibly better than anyone else has ever done it before. I mean, water runs down surfaces. Puddles form. Low areas fill up with water only when it's raining. It's immersive.
Regarding the effects on climbing, I like that it pushes you to adapt your play. Just like thunderstorms will often force you to lower your armor and attack rating to avoid getting struck, rain reduces your options for where you can go, how you can flee, or whether you can even get away from a strong monster or climb out of that canyon or hole you just jumped into. It's certainly gotten me a few times, and then in some cases I've learned to just face that monster head on or maybe stick around in that hole a bit longer to see what I can discover--that has paid off tremendously at times.
If BotW has taught me anything it's that being flexible is important. I also learn to get rid of my assumptions about the world. There's a lot to do in a small space, and even if 90% of the secrets are koroks you can sleep well at night just knowing that you've made Hyrule a little safer by exposing that thing.