League of Legends: Main support on damage, worth it or not?

We all know that there are some epic league of legends heroes that are supports but do a helluva lot damage if you build them ap or ad wise. While i agree that we can always have some fun doing that, the question is simple: Is it worth to have more damage dealt then your AD carry or AP Carry? League of legends gives you the opportunity to have builds directed towards AP but it also gives you suggestions of the items that you should have.

Ok, let's take it easy and analyze a bit a few situations where the support build is better than the ap build.
The support build on League of Legends heroes usually contain activable items that grant you the following: SighStone grants wards, Talisman of Ascension grants speed, Locket of the Iron Solari grants shield and magic resist to the champions around, Zeke's Harbringer grants increased crit strike, ability power for 8 seconds and Banner of Command that grants unit immunity to magic damage and stronger minions ( these are usually the more popular items).

Given that we have this information let's analyze an ap support or ad. First of all if the support goes ap/ad/tanky it will miss out on these great items that could keep an AD Carry or an AP Carry alive to deal more damage. What most supports don't understand is that building sighstone does take a lot of damage from your build and unless you're fed and want to finish the game quicker, there's no use to build ability power instead of support items. However you build your support with damage items he will never reach the damage that your AD Carry could have given if he has a few more seconds to live. At that point, he could have 300 damage with a critical chance of 30% and a critical damage of 800. Do the supports think that they can actually reach that damage with an already built sighstone? The only solution to reach that damage is to actually have a really long game as veigar support with a delayed damage.

I'm not saying that i don't like players that actually go damage supports. Not at all, though on the long run it is just not as usefull as a main support can be with sustain items for the carries. While League of Legends has built the heroes to actually have the power to kill the enemies, it does take a lot of the burst when you have an item that doesn't contribute.

So what do you guys think? Should you play the heroes as a sustain support or as a damage support?
Check this link for pro players to see their builds in the recent games.