Suppose HMRC claims that you are inside IR35 on a given contract? You can probably fight it, and HMRC does loose many of these cases. But in case you can't, you'll have to pay back tax, national insurance contributions, and interest and penalties on your earnings. The financial impact of IR35 is massive for contractors, and this is something you really want to avoid. Paradoxically, you will be paying all this tax without ever having received any of the benefits that employees get. No holiday pay.