Dual talent specialization is made available to players at level 40. I do not recommend purchasing the talent at this time unless you have a main toon with a surplus of to fund this endeavor. However at level 80, dual talent specialization is a no-brainer for most classes. At the cost of 1,000 gold any player should be able to afford this and the benefits are great. As of note, there is no type of faction discount that can be applied for the talent. You can purchase the talent from your class trainer.
What the dual talent specialization provides to you is the ability to switch between any 2 talent specs, glyph sets, and action bars. This talent can greatly increase your demand to your guild and fellow players. Say you have a Druid or Paladin toon, this talent would enable you to have a tank build and a healing build, which are the most in-demand roles for players. If you are a good player and have the ability to switch between these roles, raids will be begging for you to join them on their quest. Now you must have the appropriate gear to make proper use of these talent builds, as your tanking gear would not be acceptable for your healing spec. However as you progress through raids there will be opportunities for you to acquire gear for your second spec, either through raid rolls, or spending emblems.
The dual talent specialization has the added benefit of changing up the game play. The game can get stale and repetitive from doing the same rotation over and over. This talent can greatly alter your rotation or even the way you experience the same raid. Having 2 different roles with one toon could be like having a whole other toon, but without the work of leveling one all the way to 80 again. If your main spec is DPS, random dungeon queue times can be in excess of 15 minutes, however if you have a second tank spec you can get instant queue times. Most players are doing at least one random dungeon per day of playing, so this saved time can add up very quickly.
Another option for wanting dual talent specialization would be if you want to do PvE and also PvP. If you want to be successful in both of these areas you will need to have different talent builds for each area. Sure you can do PvP in your PvE build, but it is not the optimal strategy. Also one thing to consider, if you’re strictly an Arena PvP player, this would allow you to fill different roles on your teams. In 5v5 you could be a DPS role, and in 3v3 you could be a healer role. It’s comparable to the advantages of the PvE player have dual talent specialization. This use might be better suited for pure DPS class such as a mage or hunter. In most cases they will only need one spec for PvE, and as a result can choose their second spec to be PvP.
When I reach level 80 on my toons, I normally have in excess of 1,000 gold just from leveling and not even working on professions. The first thing I will spend my gold on is not epic BoE gear, or epic flying, it is dual talent specialization. The freedom to switch between roles with the click of a button is too valuable not to take advantage of.