BMI isn't completely rubbish but it is just a guide. The range of 18-15 actually encompasses a massive range of bodies. Whilst there are a few people who come out as overweight when they are just very muscular, these are actually quite few! If sprinters, who are absolutely stacked come out only around 25-26, then the vast majority of people over 25 actually are overweight! Just because BMI is not a perfect index doesn't mean you don't have a weight problem if you're not 18-25.
It is true that it tends to place the emphasis on weight rather than health based on the assumption that overweight people do little exercise.
None of us really needs an index to tell us whether we are at a healthy weight - look in the mirror! Or ask some of your friends if you're not sure about your own judgement.