All of you are asking for experienced players in off meta roles
Just like I'd expect a bronze playing Braum, Soraka, Sona or any other support to know his champion (From the 2k-ish games I've played, off meta players usually know their shit better than players picking FotM champions). You're in the same rank as me and appear to have recently left bronze as well. Unless you trust yourself way too much, I bet you'd also expect (or at least hope) that the random guy in your team knows his champion, despite of his role.
Truth be told, to make an off meta pick work the player would need more games than he'd need with a meta pick to succeed. But nothing stops it from working. Specially at low Ranks.
I particularly like this part:
It's not as great as a regular support,
Then why pick it and make it harder?
Then you proceed to answer yourself.
he does offer things the others don't.
Like?
As I said, the strongest slow in the game that is also coupled with an AS slow. Unless the enemy laner is already ahead, it's impossible for them to win a trade if Nasus' Wither is maxed. It's a 95% slow and 47,5% AS reduction.
Another cool thing is his E. It reduces enemy armor by a considerable amount (20 to 40 is nothing to laugh about, specially in the early game).
He is basicly a ball of debuffs to the enemy ADC. Sure, you might lose a trade due to heavy CC coming from the support, or have a hard time against heavy burst duos. Just like you might have a hard time against a Janna if you're playing an all inner like Leona. Nasus support has his counters and they are plenty. That's why he's not an optimal support.
He is a much more of a niche champion than most supports, but he has what might be the best supportive non-ultimate ability in the game in his W. A huge point and click ranged MS Slow + AS Slow can save the ass of your adc in so many situations. It's also a huge "Fuck You" to gankers.
In short: he can be countered, but if picked against a low burst CC + AA reliant ADC he can be disgustingly strong.
As to why anyone would pick him instead of a regular support: personal preference maybe? I have no clue.