"It seems curious to me that there seems to be a new class of worker who is treated as an employee for income tax and NI purposes but has no holiday, sickness, or pension rights"
This is the worst and most dangerous part of IR35. In effect, it allows companies to opt out of providing employment benefits to employees by calling them "inside IR35 contractors".
Being an "inside IR35 contractor" is the worst of all worlds. Extra tax is taken from you, as are many decisions related to running your company, while you get none of the benefits of being an employee (even an employee of your own company).
My view is simple: If a role is really a contract role, it will be outside IR35. If it is inside, it's an employee role and should be filled by an employee with all the rights and benefits thereof. "Inside IR35 contracts" should not exist.