Having a ladder will cause casual players to disappear even quicker than they do normally which, whether or not anyone wants to admit it, would be terrible for any game.
The casual player is by FAR the largest demographic of any gaming community. If you alienate the largest demo, you lose everything. You lose income, you lose currency flow and item flow, and the overall yield of legitimate items of any value (usability, not worth) is greatly reduced.
This however is not accounting for bots, which shouldn't be because bots have an ebb and flow that correlates with the casual player. The more casual players, the larger the demand for currency and items thus increasing botting. You may see this as a negative, but it is actually the opposite, as there are always improvements to reduce botting, which is actually great PR for any game which in any case is a postive and will actually increase the amount of casual players thus increasing all items we have mentioned previously. If you can create a game that has 0 bots, then the community is right back to the greatest demographic, the casual player.
This is in no way condoning bots. Bots are unfortunately a permanent part of the gaming community, which if any gaming company is smart, and if you have any intelligence you would have also noticed, they will keep a very tight leash on, and because they will always be around, I'm sure that every gaming company has learned to use them to their advantage, such as Jagex did with there rather recent anti-botting exploits.
Kind of got off of the main topic however...ladders would be very bad for this game.
TL:DR ladders suck