Well, I'm not deaf, and I don't personally know anyone who is deaf or hard of hearing, so I can't answer from my own experience.

That's no problem though, because a quick Google showed that there are loads of deaf people who benefit from having a service dog to support them to hear. I have no reason to doubt all of these people who find that this helps them, so I feel confident in answering your question with a "yes"

You might feel that basing my answer on a quick skim over some search results is a cop-out answer, but that's the result of quite a few years being active in disability activism and reading disability theory. A slogan that's common in disability rights activism is "nothing about us without us". That's the crux of things — I don't know what people find helpful, and it's not my place to tell people what is helpful to them or not.

For me to question whether someone who has a hearing ear dog "really needs [one]" would be like asking a wheelchair with multiple sclerosis whether they really need that wheelchair. Even if I had multiple sclerosis myself, that would be highly inappropriate to ask, because disabilities affect people differently, and people develop coping strategies that work for them .

I've been on the receiving end of questions like that, and it's not pleasant to have to justify yourself to a person who has little to no relevant expertise on your disabilities like this.

Living with disabilities often means trying to put together a wide variety of accessibility adjustments that may help, but all come with a tradeoff. Using myself as an example, I'm an ambulatory wheelchair user — I use a manual wheelchair sometimes, due to pain and frequent falls, but I can walk (albeit riskily). If I take my wheelchair out with me, then it makes it a bitch to get around if the venue or the city isn't particularly accessible. If I have recently fallen and sprained my hand, I can't push myself, so that's awkward. Sometimes, the path of least harm means taking a risk and trying to walk (usually using a different mobility aid, such as a walking stick or frame). Walking where possible is also good to prevent muscle atrophy.

Do I need a wheelchair? Well, I guess not, given that there was a period of almost 2 years where mine was broken and it was a nightmare to chase up wheelchair services to get them to repair or replace my wheelchair. I survived that period, so in a sense, I don't need it. However, my quality of life was significantly decreased, because on the bad days where I was too unsteady to safely walk at all, I was housebound at best. I fell over far more frequently, and I ended up making at least 20 trips to the minor injuries unit for X-rays following falls in that period. It sucked, big time. And for those reasons, I was eventually given a replacement wheelchair, because the specialist services agreed that I did need one.

When you see a disabled person, you don't get to see their behind-the-scenes. If you're seeing them out and about, there's a decent likelihood you're seeing them on a good day anyway, so there's a sample bias there. You don't get to see what their struggles look like without their accessibility adjustments, nor the struggles that remain even after they have all their support in place. You don't get to see the costs or the tradeoffs of their accessibility adjustments, nor which strategies they tried, and then decided against using because it wasn't helpful for them.

Understanding this, in broad strokes, is why I'm inclined to give someone the benefit of the doubt. I'd rather let some "fakers" get away with claiming accessibility adjustments that they don't need if it means supporting disabled people to figure out what works best for them — not least of all because there are far more disabled people struggling needlessly due to not being able to access things that would help them, than there are "fakers".

TL;DR: Yes. Absolutely, without a doubt. I don't have lived experience of being deaf, so if I want to be a good ally and advocate, then the first step is to listen to what people say helps them, and believe them.

Did you read the comment you're replying to? They already said it's not your place to decide who does and doesn't need a service dog. They didn't say that to mean that it's actually their place to do so.

Many of them already are used as service animals.