They still count towards a prime engram and a lot of them are really fun. That's two good reasons.

This is a hard one for me to make a decision on. They have tried shifting powerful rewards towards newer activities in general, which I think is a good thing, it was boring over the past year to constantly need to run old content before new content in order to level up. Keeping the Flashpoint is a good idea to grant incentive towards visiting old destinations, but Adventures kind of fall into that same category? Hard to say. For the Heroic story missions, they were replaced with a weekly story mission on the moon for a powerful engram, that's a pretty good change. I think the reason the Adventure still isn't a powerful is because they seem to be moving away from Adventures and more towards other types of activities such as Nightmare Hunts and Vex Offensive. All of that being said, I think it is okay they removed powerfuls from Heroic Adventures. Maybe a compromise would be to have the first Heroic Adventure per week give more progress towards the Flashpoint, maybe like 50%. Although that being said, what do they do when the Dreaming City or the Moon become the Flashpoint? This may be another reason they removed it.