You have broken your ankle and it hurts but you can still get about on crutches. How long until the doctor suggests that you get rid of them and start walking unaided? Hopefully as soon as is sensible because that will help you recover strength in the damaged area and not using your muscles properly (or at all) will lead to other problems. Not to mention it is just better to get on with your life as normal.
Chairs that support every single part of your torso seem to go against the notion that crutches should be temporary. We are designed to be active and take our own weight while we forage and hunt and although most of us are less active than our ancestors we are perfectly capable of supporting our own weight. Some may have specific problems that need extra help but the final goal would be, I hope, to get everyone back to normal – ie without crutches.
There is a movement to get more us to stand more particularly if we have desk jobs. Their website is http://www.getbritainstanding.org. Finding a balance between sitting and standing has to be the key and using a chair that encourages you to support your own weight and keep moving seems sensible to me. So if someone suggests that you need back support when sitting ask them how long you will need it for. If they reply ‘Forever’ then maybe you should ask them how they can help you do without it in the future because that is what you would ask the doctor giving you crutches.