The obvious answer is it depends on what your needs are, but with how rugged and light optics are getting these days I’m beginning to think it’s time to move away from irons on hard use rifles. If you are using an Aimpoint or ACOG in a high quality mount it would be more practical to use the weight to carry a spare bolt and extractor, as the chances of those failing are equal if not higher, and more catastrophic.
Let’s suppose you are running an ACOG, or another rugged magnified optic like a Schmidt and Bender Short Dot. If it failed and you were forced to resort to your BUIS you would have to remove the scope with the 1 or 2 throw levers that secure it, flip up your front and rear irons (or just rear if you have a fixed front), and reacquire your sight picture, all under stress, possibly taking fire. Alternatively you could have an Aimpoint H1 or T1 in a 1 o’clock style mount, allowing you to slightly tilt the rifle and have an aiming system that’s much faster, and far more suited to the kind of close quarters and low light situations where you need a backup the most. An H1 or T1 with mount weighs 105 grams, a set of Troy flip up BUIS is 110 grams. Combined with the battery life of the Aimpoint, if money isn’t a consideration and my life is on the line I take the T1 in a cant mount over the irons in a second.
Maybe you aren’t a high speed operator though, and aren’t using thousand dollar optics, are irons necessary then? Nothing is really necessary for a range gun, but it’s nice to have a second option if your $200 scope or $50 mount disintegrates. On the other hand how much spare stuff do you really want to bring to the range? If your setup is that liable to break it may be smarter budgetwise to buy higher quality in the first place, you may end up spending more on emergency replacement parts than you would have a bombproof part in the first place. A set of quality irons isn’t cheap, from a purely practical standpoint I feel the money would be better spent on getting a higher quality mount or optic if possible.
All that said I do have irons on my rifle. Why? Because I like shooting with them. You may feel that a few ounces and a few hundred bucks is worth the enjoyment or peace of mind too, even if it’s not the most practical purchase for your rifle. There’s also the “why not” of it, they aren’t heavy or bulky enough to really have anyone think “man, I wish I never put these on.” For these reasons, and maybe even tradition, I don’t expect to see irons disappear from too many rifles in the near future, but the days of them being necessary are over.