I can say that at my last company, redesigning the login page was bottom of the stack of things we wanted our first designer to work on. We just had a centered "Login with Google" button, our logo, and a email + password form. What else do you need?
Simple it good though, agree with that.
In particular, those customers that think adopting an early-stage prototype will give them a competitive advantage, they expect something that's not well designed. If they get something shiny when they've asked for a prototype, they'll be worried you're trying to hide a turd behind the polish, whereas if you give them something that doesn't look great, they'll know you're serious about giving them the latest, rawest, earliest experience of whatever it is you're innovating on.
(Of course, "early adopters" is a rather small market niche, so you might not grow very far doing just this.)
And then sometimes bad design is just great on its own merits. By way of example: https://x.com/NCTreasurer/status/1577673238536245248?s=20
not surprised at all
The very first prospect we had put his hand on the railing, looked up at the house, and said "this place seems to be well taken care of." And made a decent offer the next day.
So I'm in favor of detailing the sign up page with care.
If the login page is simple and generic, the message is way toned down. Similar to the advice of not polishing the front-end too much if the backend barely works because clients will expect that a finished interface means a finished product.
For B2B products it's a little different. When you're holding an MVP in your hands, it's really hard to tell whether you're grasping a rusty old anchor that will drag you to your death, or whether you're looking at lightning in a bottle.
One quick way to tell is if businesses sign up and user it even though it's as fugly as can be.
If, when pitching the product to a potential sale, the subject indicates that they need particular cosmetic changes before higher-ups would approve it, then you're likely pitching a dud product.
If, otoh, they interrupt you during the pitch of that fugly product, and offer to trial it immediately, or buy the prototype, or call in a higher up, then you have the proverbial lightning in a bottle.
Cosmetics is, I think, a good indicator for B2B products - if people want it even though it's ugly, you're selling painkillers. If people want it only when it's pretty, you're selling vitamins.
Lots of people make a quick judgement on your trustworthiness based on whether you at least have adequate design. If they turn away in 200 ms because of poor design, they will never be able to evaluate what your true value proposition is.
Many decisions in a B2B context are defensively made, so as to avoid the risk of looking stupid in the future if things don't work out. Like the old saying, nobody got fired for buying IBM. If your branding and design isn't at least adequate, again there will be no trust and a fear of being blamed in the future.
I've even had clients tell me that design really doesn't matter and they just want to see functionality right now, only to be shown something with full functionality but poor design and then walk away.
The trick is to not be a perfectionist but to spend the 20% effort that will get you to the 80% which is good enough.