There's a number of subs that are actually a lot more effective than /nin that I've found. Especially in the lower levels where a lot of whatever /nin has going for it is negligible, or even not in the picture to begin with.
The problem I see on most Dancers is the fact that the community has brainwashed Thieves into thinking that DW daggers is the only effective method to do damage. This is wrong. DW Daggers is good, but not the only method; there are other, equally viable methods and subs to do what you need to do. Because Dancer and Thief are extremely similar in gear options, many people have just adopted /nin as some sort of standard. Nothing wrong with it in and of itself, but there are indeed options that are equal to or better than /nin.
No, having shadows isn't an excuse to let healers be lazy. No, using the "MP efficiency" excuse isn't valid either. It's Dancer, they can heal themselves in a number of ways that won't jack up hate.
As for subs that work, here's a few: Warrior, Thief, Monk, Dragoon, Dark Knight, Samurai, Blue Mage, Corsair, Bard, Puppetmaster, Beastmaster, Ranger. All of those offer something in terms of either Job Traits, Abilities, related gear, or all three, that put them on par with or exceeding Ninja.
Here's an example. Let's take Dragoon. Yeah, we don't get the Wyvern, boo hoo. Not an issue. The point of this sub is to enhance our Attack, Accuracy, and Base Attribute values, while giving us additional TP sources to work with for our dances. There's even gear that works with this sub specifically to enhance some of those abilities through allowing faster attacks* and even further Attack boosts. Since you'll only be swinging one dagger, the fact that that dagger will be hitting harder and more often offsets the fact that you're only swinging that one dagger.
The additional 5 Attack and Accuracy from a second dagger that gives those boosts kinda isn't that good compared to having a full +10 of each just by changing subs, while opening that slot up for other equipment. Also, while you're at it, why not take advantage of /drg and toss on the cape? Another 6 Attack right there. More Attack is always good; gives you a better chance to hit for more than 0.
Jump is also another bonus for extra TP as well as a bit of damage.
That's just the example for Dragoon. There's quite a few more jobs up there that all have something of worth to give Dancer as a sub. Get out of the "/NIN ONRY!!!11!" mindset, and you'll find some very good combos
that actually work out there.
*