Yes, it’s a doorway into Buddhism, but not Buddhism itself. There are other doorways into Buddhism, I believe many in the west stumbled upon Buddhism via psychedelic drugs. Psychedelic drugs are not something I would recommend.
The issue with some secular Buddhists is if they claim that secular Buddhism is a form of Buddhism, or worse, the original Buddhism as the Buddha meant for it to be. Then it becomes a cult. Something which claims the name of Buddhism, but serves the food, with the essential ingredient missing. There are plenty of other cults of Buddhism Controversial ‘Buddhist’ Teachers & Groups (viewonbuddhism.org) which we wouldn’t necessarily recommend people to get into, although, for some cases, it’s good for them to get exposure to Buddhism and come to right view eventually. Best is to come to right view straight away.
It’s still ok to use secular Buddhism as the doorway to ease some people into Buddhism, but basically, we are already doing it by saying: leave rebirth aside for now, let’s focus on meditation/psychological parts of Buddhism. The term secular Buddhism gains a dirty name due to some of them not being willing to see secular Buddhism as merely a step along the journey to Buddhism, but as the endpoint in their understanding of Buddhism, not open to be converted.
It’s an issue for the sake of future generations. Who knows if the current mainstream Buddhism which includes Mahayana initially started as a splinter group which is universally condemned by others, then over time, had to be acknowledged as mainstream for the sake of harmony? I am thinking of Nichiren Buddhism, which looks very different from Early Buddhism, yet is now recognised as one of the mainstream Buddhism in Japan. It has very similar practises to Pure land, which is on an even more solid foundation as mainstream.
Could calling it out as not Buddhism work? Look to the case of Lu Jun Hong, Lu Taizhang of Oriental Radio. I don’t foresee them becoming mainstream Buddhism, perhaps due to so many orthodox Buddhist organization calling it out as not Buddhism, or as a cult. However, secular Buddhism has the potential to become mainstream. I dunno if calling it out would ultimately work, but it’s better than just keeping silent as if we approve of them modifying the teachings of the Buddha to suit materialism philosophy.
For the sake of the practise, kamma and rebirth does form a major part of the morality basis in Buddhism. Secular Buddhists use humanism as their morality due to not believing in kamma and rebirth. There are one or two part which is of the crucial difference between the 2 morality here, which might become relevant.
Say for a depressed, single, secular Buddhist, who’s suicidal and doesn’t have any friends or family or relatives. They might not see anything which would prevent them from killing themselves, since there’s not even anyone who would be sad for their passing. Since they don’t believe in life after death, to them, suffering can end faster by killing themselves, rather than to keep on living without hope, suffering like hell in depression. Whereas, for us Buddhists, we would avoid suicide at all costs due to kamma and rebirth. It might not be that easy to convince a secular Buddhist at that stage about rebirth, but more conventional anti-suicide techniques have to be used.
A second issue is euthanasia. There are plenty of posts by secular Buddhists, or new Buddhists from the west (who most likely doesn’t believe in kamma and rebirth yet, or haven’t internalized it) that says that they are ok with killing animals who are fatally wounded, dying slowly just to quicken the death of those animals. They don’t consider the bad kamma of killing they would incur on themselves. They don’t consider that ending the animal’s life doesn’t necessarily end their suffering (due to samsara).
This becomes even more important when it comes to parents. If for some unfortunate cases, some people’s parents got into a terminal illness, no hope for any quality of life, comatose etc, which at some point might face the decision to be hooked to a life support machine. If later on, due to various reasons, the children has to decide if the life support machine would need to be unplugged, that’s killing parents there. 2 of the 5 heavy evil kammas, resulting in hell for sure next life, and no attainment possible. For those who don’t believe in kamma and rebirth, there’s much less incentive to stop them from that unspeakable evil.
This issue would be multiplied to generations of unfortunate Buddhists if secular Buddhism became mainstream down the generations.