First, your post is misleading as Discipline in TBC is just as much healing oriented as it is damage oriented.
Second, Discipline is an excellent choice if the person leveling the priest intends to do dungeon content while leveling, especially at level 40 and beyond. The value of having access to a player above a certain level threshold, but also below max level, who is in a spec with talents for healing is quite lucrative due to how problematic it is to heal in a healer spec.
So, with all of that said, if proper talents are chosen, a Discipline spec priest can be even more efficient than a Shadow spec. More efficient in healing, damage, sturdiness and mana. Essentially more efficient than Shadow in almost every way. It is also important to remember that Shadow is not technically classified as a "DPS" in TBC like it is in later expansions, but rather it is classified as a "mana battery". The DPS the spec can manage is decent, but that is not the role of the spec. The spec requires a bit of gear to properly out-class Discipline, and most certainly requires spells that are earned very late in the leveling process.
End note: It is important to note that there is a very large and discernible difference between WotLK Shadow and TBC Shadow. This is a point of contention considering your post history revolves primarily around WotLK.
@vkhi, leveling as Discipline is advisable, especially if you want to easily find groups for dungeons while leveling. The only real advantages Shadow has over Discipline during the leveling process is the silence talent, and the ability to learn how to play the spec while leveling. If either of these things are a must-have for you, then I would advise that you play Shadow. Otherwise, I would recommend that Discipline be the spec of choice. If you do decide to level as Discipline, make sure you get all of the talents that empower your Power Word: Shield - especially the reflective shield talent, and make sure you get all of the mana efficiency talents - including the ones in the first row of the Shadow tree (which can be obtained later).