Newcomers ask 'do I have to buy a 'uniform.' In our dojo the answer is 'no.' We find that over time people choose to. I responded to a new inquiry with "we don't obsess over finery or nit-pick over obscure rules. It is obvious when someone is being disrespectful, not matter if they are well dressed and following the letter of the rules. Just come Play Aikido and over time you will learn the details through experience and observation."
I think the gi is a pretty sensible garment for aikido practice, personally. You need something that is going to protect you from the mat and is going to give your training partners something to grab onto. The gi works really well for this; I've certainly never seen anyone come up with anything that works better. What's more, although I don't think this something essential to or definitive of aikido, I think there is some merit to the argument that our purpose in the dojo is to be part of a group rather than to declare our individuality, and that therefore we ought to dress accordingly. For these reasons, I don't have an issue with clubs that make their students wear gis; I think there is more to gis than just tradition for tradition's sake.