No one will ever question that Shaman's in either Elem or Enh are GREAT DPS, some of the highest in the game in many cases. So its never a question regading the "effectiveness" of a shaman dps, but rather a question of raid/party "need" that shoe horns us into healing roles.
Regarding endgame raid/heroic dynamics, finish the end of the following sentences;
Rogues are good at dps, situational CC and _____?
Locks are good at dps, situational CC and _____?
Mages are good at dps situational CC and _____?
Hunters are good at dps situational CC and _____?
See the trend? Fact of the matter is MOST classes are DPS, and PURE dps at that. While its more than respectful that shaman's can play with the PURE classes as far as #'s are concerned, its the simple fact that DPS is a dime a donzen, and typically don't make or break a party/raid wither a party/raid is going to head out and due their thing. But lack a tank or healer...and your going NO WHERE!
This being said, shamans CAN heal, and few classes can do so (and ever fewer can tank btw), thus by default we are PUSHED to spec healing when in comes to endgame simply based on whats "needed" more.
Now, that doesn't mean that you can't dps, but I'm sure this enigma is similar to DPS warriors pushed to be Prot, or Shadow Priest pushed to be Holy or Dics. I think the only 2 classes that "tend" not to be pushed out of their "lvling/grinding" specs in order to suit endgame needs would be Feral Druids and Holy/Prot Pallys...but thats a different topic.
Now, I happen to be a DPS enh shaman, that does (actually I should said "did", before my RL schedual changed) raid as a 70 and have done so for a rather long while now. I tend to be at least on the top 3 in total dmg done, thru out the raid run, and typically in the top 5 in total healing done. There's been a recent elem sham that came to our guild, and he tends to be in the top 3 total dmg also (and DOESN'T EVEN MAKE THE LIST on the healing Done 0_o....noob!).
So shaman's DO put good #'s, but here's how I look at dps shaman's endgame. Putting up good #'s...well frankly thats easy! Just get good gear that supports dps stats that are associated with your spec, stay alive and go to town :P Here's the hard part, that MOST dps shaman's DON'T do. Where pretty much the ONLY class that can do BOTH dps and healing WHILE in combat, so play accordingly! A enh shaman JUST meleeing, is nothing more than a Rogue without a vanish, a elem shaman is nothing more than a mage without ice block. That being said, your worth can be looked at as such, thus REPLACED by either accordingly! SO what can you do, that other classes CAN'T!?
This all being said, no one can tell you to spec what THEY want...person that honestly pays their months can do whatever the HELL they wanna do :P If your on your parent CC, then please delete your account, save your parents money and go play maplestory or something free :p But, I digress...
Play what you feel comfortable with playing, but understand in a guild/raid/party setting DPS'ers aren't something typically "needed", but healers are! And that lazy rogue in your guild thats "lol" @ a dps shaman, is just too lazy to reroll a priest himself :p