I don't find it rude, I always struggle to pick out gifts, so when someone tells me what to get, it makes it easier. However, other people find it rude because gifts are supposed to be surprises and not asked for, because once someone is asked to give something specific, it basically means that you have to get them that thing (like money).
You really shouldn't ask for anything for your birthday unless someone specifically asks you what you want.
People offer gifts to you on your birthday as a way of acknowledging the day you were born...nobody is obligated to shower you with money or gifts just because it's your birthday and it's selfish to expect it.
Some people enjoy the buying of the gift, but i don't think its rude if its a hard to find item or something you would have to pick up.Another way to do it, is have them go with you. or if the do give you the money make sure you give the giver an update or show them what you bought to make them feel good.
no, its perfectly fine. if there going to otherwise spend money on you its easier to give you money so you can pick soemthing out yourself. its not a very personal gift, but if they dont normally or cant get you things you like, its easier