Necessary? Depends. They're going to continue to grow and will become a bigger issue later. if they're going to cause trouble in the here and now, get rid of them. if it's not going to be an issue for years to come, then no big deal.
Are not causing symptoms? well i guess that they haven't enlargened yet and/or moved around in your gall bladder. Gallstones hurt like crazy and in the end, may completely destroy your gall bladder and not allow any storage of bile. Gall stones removal is extremely necessary
yes of course,they should be removed ,because the presence of the stones for a long time in the gall bladder will irritates its wall –>changes in the epithelium ( metaplasia) –>possibility of development of adenocarcinoma of the gall bladder (cancer) which is a very bad one 
MD opinion
Yes, but you don't have to have surgery to remove gallstones. and, this also indicates that your liver is full of gallstones and you need to do a liver cleanse, soon. and, they DO NOT form in your gallbladder, they form in your liver. since the liver is a very important filtering system of your body, you do need to get rid of the stones. it is very simple for you to learn to do a liver cleanse and get rid of all of your gallstones. But, over a period of time we all form more stones in our livers so it is necessary to know how to remove gallstones, naturally. This is one surgery that is done all the time, most of them unnessarily by medical doctors. the key ingredient that you need in removing gallstones is epsom salt. I learned this procedure from Dr. Hulda R. Clark's book, "the Cure For All Diseases." She gives the procedure to follow to cleanse your liver of all of it gallstones. and, yes most doctors will tell you that gallstones, are formed in the gallbladder, but they are actually formed in the liver. But, the bigger ones can get caught in one's gallbadder by one not cleansing the liver. See the link below for Dr. Clark's book and save yourself the time, cost, and the suffering of having an unnecessary surgery.
Yes. it can lead to future ailments. Cancer too.