That does suggest a potential problem and it's something that might want to be looked into.
But then where's the cut-off point? That's the issue your argument runs into.

Lets pretend Random Idiot has a completely off the wall build. He takes forever to kill things. He dies a lot, but eventually gets there. He now wants to join your group.

Do you say yes or no? No, right? Why? Because his build is bad. It's not efficient. It wastes your time.

At what point does SE say, "No, your build simply does not work." There has to be some point. If there is no cut-off point, then bosses will have to be so easy that Random Idiot can go kill a boss only using Aero 1 if he so desires. And who are you to tell them otherwise? The skill system is supposed to be free, right? But wait, you do care. Why? Because while Random Idiot is struggling with the fight because he's spamming Aero 1, you're sitting there bored to tears because it takes no effort for you, the min/maxer, to actually defeat the boss.

but this is the thing, you are in NO WAY required to have the skill to win, it just makes the fight more difficult
I will admit that I have no experience with this fight. So maybe you can fill me in.

There's a difference between "not required" and "not required, but it sure makes things a lot easier if you do." If your odds of completing some portion of content are increased significantly by using a certain skill, to the playerbase, this becomes required. In MMOs, time is king. Time trumps skill in a lot of cases. The best use of time often becomes the requirement or the expectancy.

Now, if your argument is that the fight without Sentinel is only marginally more difficult (or is a crutch for dumb players) and the playerbase is blowing things out of proportion, then you have a point. Regardless, I still think my point that open skill systems are a slippery slope remains, though perhaps not for this particular issue.