It is the most irritating thing and one of my main reasons for wanting a homebirth - well, one of the many reasons, hospitals do such an incredible job of making you as imobile and uncomfortable as possible.
With my first LO, they sent a student midwife to insert the IV, she made a real cack job of it, and I spent my entire labour and a good many hours afterwards with my hand covered in dried and caked blood and this awful needle sticking out of it, it isn't painful, just uncomfortable and you are so conscious of not wanting to knock it on anything. ouch, that would hurt.
I had the same issue with my second. For my third birth, I was in a French hospital where they are even more medicalised, I was SO lucky to have a wonderful midwife who told me that though it was LAW to have an IV drip to rehydrate you, she would make an allowance for me and allow me to have it after the baby was born, so I could remain mobile during labour. I was still strapped to monitors though, which is also a royal pain in the ass, but I respect that is more to keep a check on LO.
I suppose they leave the IV in, just in case they need to use it again afterwards, but imo I think it is a little unecessary in most cases and not very fair to the new mums, as I am sure in most cases it is never used again, and they could always insert it in the other hand if something came up later and there was an emergency. It really is just super uncomfortable. I am so looking forward to a home birth, where I am the boss and not subjected to all that equipment and all those people walking in and out!