hexy steroids are usually pretty 'last-ditch' and I am also surprised that they are considering them when you are not in hospital, but I am guessing that they are concerned about your weight in particular. Unless you are extremely tiny, 7st is very light indeed and although they don't really know why steroids often help with hg, one thing that is definite about them is that they increase your appetite. I was on them for a really long time (from 7 weeks to about 20, at varying doses) as they were the only thing that really helped this time. I found that unlike almost all the other drugs they actually helped the nausea as well as the vomiting, and most of all they helped me feel like eating at least a tiny bit. This was particularly important for me as I had lost too much weight and couldn't eat enough to get out of ketosis. Even though this pregnancy has actually been a lot worse than my first in almost every way, I have never totally lost confidence in eating this time which I did last time - and didn't get it back until months after DS was born. I think this is partly down to the steroids.
How far along are you? There were historically concerns that steroids might interfere very rarely with the very early stages of the baby's development, but that concern has not been replicated in any more recent trials, all of which suggest that they are safe for the baby even in the first trimester. For comparison, the same concerns were raised about ondansetron at one point, but again, all the recent studies have been reassuring, and steroids have been used in pregnancy for much, much longer than ondansetron. I was actually more worried about the possible long-term effects of the ondansetron (especially for me) than the steroids, for just this reason.
If you are 8 weeks or more they are almost certainly completely safe. Other specialities (e.g. rheumatology, asthma doctors - like you I also have RA and have been on steroids for that before) are much more relaxed about steroids in pregnancy and often consider them the safest option - it's quite odd but obstetricians are generally much more cautious about them than other specialisms.
There are recognised risks to steroids, especially if you end up on a fairly high dose for a while, but they are risks to you, not the baby - of bone wastage, muscle weakness, and a considerably increased risk of developing gestational diabetes. You may find that if they start you on them they will also want to test your blood sugar regularly. If you have ever had any mood disorders (e.g. a serious depression, or a manic episode) than you also need to keep an eye on this as steroids can destabilise your mood a bit, especially in the higher doses. (Though in practice I found that they just slightly counteracted the incredibly depressing effects of hg, so I was quite grateful for that!)
There are different protocols for using them, but it is quite likely that they will try you on a very short course, starting with a fairly high dose and then tapering down very quickly to nothing over only 5-7 days, and then see if that is enough to improve your condition even after you are off them. If they help to start with but you lose control of the sickness again towards the end of that taper (which is what happened to me - I was back in hospital after only two or three days out) then they may put you back on them again.
There seems to be a lot of difference between individual doctors re: steroids for HG, some are very against it (I had a massive ongoing struggle with my first consultant, even though absolutely everyone else, including all the nurses, thought it was obvious that they were the only thing that was helping), others very pro. My father is a retired doctor and he looked up all the studies for me and was reassuring about them. But if you feel that you are managing OK without them, I think it's fine to ask them to explain in more detail why they are recommending them at this point. If it mostly to do with your weight loss, you could also ask to see a dietician for ideas.