So, space marines... (warning, I'll rant) :)

First off, and I admit this is pedantic, a Rhino was AV11. So Land Raiders and Monoliths would still be tougher. Much more importantly, though, there was the issue that Dark Lances were crap against AV12. A 1/3 chance to penetrate a vehicle was not something an anti-vehicle weapon should be boasting about. It meant that even a Ravager with a full compliment of Dark Lances still wouldn't average a single penetration (since it will only hit with 2 on average).

What's more, people ignore the opportunity cost of having the Lance rule. Again, I bring up the highly-comparable Lascannons, which instead get +1S and +12" range. Lascannons were worse against AV14, the same against AV13, and better against everything else. And that last part is key, because the vast, vast majority of vehicles in the game were in the AV10-12 range. You know, the range where the Lance rule was worth absolutely nothing.

The perception of the Lance rule has always baffled me, frankly, as people seem to regard it as being about 90% more effective and useful than it actually was.

Anyway, that aside,, 'crew stunned/shaken' was not a good thing to aim for. Not least because it became less and less reliable, with more and more vehicles able to shake off or outright ignore such effects. Not only that, but the glancing hit table was subsequently removed entirely, and the penetrating hit table changed such that the AP2 of Dark Lances was worthless. This made it even more likely that even a penetrating hit would only score one of these useless results.

And this is before we get into Knight territory, which also ignored Weapon Destroyed and Immobilised results.

Now, I will grant that Dark Lances weren't too bad in 5th. They still weren't great and fell well short of their supposed ability, not to mention their perceived reputation. However, it was the subsequent editions that really should have hammered the nail home as to just how useless the Lance rule was.

@Barking Agatha wrote:

Second, we had a lot of them. Who else could bring that many S8 AP2 lance shots? This was before all of this "S7 Heavy10 Damage 2" nonsense.

I'd wager a lot more races than you'd think.  

But here's the thing - spamming Lances was not a good thing for Dark Eldar. it wasn't something players necessarily wanted to do, but rather something they *had* to do because their options were 'spam lances' or 'play a different army'. You know how I said above that the Lascannon was better than the Dark Lance against the vast majority of vehicles? That wasn't even the real kicker. No, the real issue is that Marines, IG etc. didn't rely on Lascannons to destroy enemy vehicles in the first place. Lascannons could destroy vehicles, but, like Dark Lances, they couldn't do so reliably, even when spammed.

Hence, those races just used Lascannons to soften up or delay vehicles. To destroy them, they used stuff like meltaguns with AP1 and 2d6 armour penetration. These are the sort of weapons that would have been invaluable to Dark Eldar during 6th and 7th edition. But since the best they could get was Heat Lances (which weren't even available to most infantry), they were stuck plinking away at hull points with ineffectual and overpriced Lance weapons.

Long story short, for all their faults I don't think it's wrong to say that Dark Lances in 8th/9th are still the most functional they've ever been.