Stables have their uses... mainly when TG are close to maxed out, or for Halfling attackers due to how their army is set up. They were good even before specs and elites were devalued, if they were used in the proper circumstances. This is why the TG vs. Stable argument is silly, and really has no place... it's not too hard to compare TG and Stables in terms of their offensive power, but the difference is usually either slim, or the breakoff point where either is not worth building is obvious with simple calculations. The questions are...

-whether Stables are worth sacrificing other building types, and whether the economics of ponies - good and bad - are worth it in their own right, as a source of raw offense
-on a more general level, how is that extra offense useful, aside from saying that you have that extra offense.

Stables have some serious economic downsides, but some upsides as well, being the cheapest source of raw offense available and not taking population. The downsides are a bigger factor for kingdoms that use pump strategies, and the raw offense upside is less relevant for superpumped offenses.

re: pony theft - it's a very good thing in hostile if armies are caught at home, and possible to use in war. It is a very effective way to weaken enemy offenses, but using the stolen horses is trickier. It isn't a reliable substitute for stables, but it is definitely a factor against building them. For this reason, if using stables then it is important that the mounted armies take initiative in hostile whenever possible, or keep their armies out on random grabs. This is not always practical.

The most important value of stables isn't offense for the sake of offense, but whether that offense translates to tangible results. On that basis, the best races for stables are the races with attacking bonuses like Orc and Undead, since they get the most benefit from each extra point of offense available to them, regardless of what that extra offense is proportional to their raw offense from other sources. Every other race will usually have considerations besides offense that play better to their racial advantages, and thus their building choices will be different; even if those races were playing for better offense, they will still get less utility out of stables than a race with attacking bonuses. Finally, the reason for that extra offense can be to put offensive power up to another level, so that faeries can be broken and double-taps are more frequent. A race that is just building stables to have average offense is often in a position where they can attack fine without the stables, which makes the benefit of that extra offense dubious.

Ultimately, stables are fine in their current implementation. If they are tweaked in the ways that some have proposed like 1.5-2 times capacity, they would become a mandatory building again for anyone who wants to be a serious attacker. At the moment they are effective enough without being overpowered, useful for certain purposes. It is largely due to the playing style at the top that they get short shrift, though at the lower rungs of the game kingdoms either use stables correctly, don't use them out of ignorance, or rarely use them for silly reasons like TG/stable optimization.