sweetheart Underwires are absolutely fine as long as the bra fits correctly so that the wires don't sit on any breast tissue.
I'm pretty sure this method isn't being promoted by manufacturers as it's not financially viable for many of them to stock a proper range of sizes!
I've cut and pasted this from another blog but can't remember which one!
Why don't more stores measure and fit properly and stock a corresponding range of sizes?
It's because of costs for the manufacturer.
If you carry 32-40 A-DD, that's a grand total of 25 sizes. So, you only have to have your designers come up with bras for 25 sizes, in maybe 3-4 styles. Then, using this, you can make all of your bras, swapping over moulded cups and under wires from sister sizes (e.g. 30G/32FF) to save even more money. Then shove the majority of the population into these sizes, and presto, you're making huge profits. Plus, it's easy for the fitters (everyone knows "A is tiny, DD is enormous").
But, if you were to be a super-awesome, carry everything store (like Ewa Michalak, who will custom size pretty much anything), and carried (for example) 24-50 A-LL, you're looking at 14 band sizes and 18 cup sizes, or a massive 252 sizes, and with the same 3-4 styles per size, you're making a literal metric shit tonne more bras than your competitor. And you know what? Your designers (who are being paid by the hour) are working a lot more than theirs (so you're losing money there), and you have to spend more on supplies (so you're losing money there), and not all of those sizes even sell well (and get recycled every season, so you're losing more money there). Plus, you have to actually educate your fitters, to figure out how to tell which bras fit, rather than just "supporting and giving cleavage" (which is even more man-hours wasted, and anytime you have a fitter who doesn't know their stuff, you have to do damage control because you hold your staff to a much higher standard). Not to mention a lot of properly fit bras don't give anywhere near the level of cleavage a massive band/tiny cups would - so girls are going to want the huge cleavage-y look, leaving you with - nothing.
Which is quite likely why Ewa is their own brand and store - if they tried mass-manufacturing, it probably would bankrupt them. A=tiny, DD=huge, 32 band = less than 115 pounds, 34-38=normal, and 40+ = big tits or fat. We know this isn't true here, but how do you convince millions of women that their information is wrong?
Bras are an intimate thing - like one guy I saw put it, can you imagine trying to tell a man he's wearing the wrong underwear, because his balls sag, and he gets a flap of skin pushed to the side? Why would you expect to have women respond better to a perfect stranger telling them all that they know about bras is wrong, and that they now magically have "H cups", which is a size only (badly fit) pornstars use?