There is an important difference between tanks and tanky DPS, and I've noticed that some people, thinking that certain tanky DPS are tanks, take them to fulfill that role.
Although some team-comps do not require a tank, others do. In these situations, a tanky DPS is the WRONG CHOICE... and it's time for people to realize that.
1. What is a tank?
There are two main purposes for a tank; one is to initiate, the other is to protect your squishy mages and carries. Sometimes a tank can do both, as in the case of Galio, whereas sometimes they can only do one, like Rammus or Malphite. Tanks are often ignorable in terms of damage, but make themselves an issue in the fight by delivering CC all over the place or by reducing the attack capabilities of the other team (think Malphite's ground pound).
Since they are not building damage, as they tend not to scale well, and since they are often initiators and bush checkers, tanks build survivability and extra CC items such as Frozen Heart and Randuin's Omen.
2. What is a tanky DPS?
A tanky DPS has one key similarity to tanks; high survivability. However, a tanky DPS builds survivability for a different reason; rather than staying alive so that they can stick around the carry for longer and dish out more CC, a tanky DPS builds survivability for a different purpose; either because building survivability directly benefits their abilities, such as Olaf building health, or because the damage that they deal is optimized if they survive for as long as possible, sacrificing possible burst damage.
Tanky DPS may have CC, but not necessarily; in fact, they usually don't.
3. Who is a tank?
In alphabetical order, the tanks that we tend to use are Alistar, Amumu, Blitzcrank, Cho'Gath, Galio, Gragas, Malphite, Nunu (although he is more mage than tank), Rammus, Shen, Singed, Taric (also magey), and Xin Zhao.
These champions have either excellent initiation or carry defending abilities, and all carry at least one type of CC. Some of them only have soft CC, but make up for it in other ways, such as Gragas and Nunu; although they have no stuns, they have ults that completely alter team-fights plus the ability to buff their own team/debuff the opposing team. Although they of course want to survive, and some of them (particularly Xin Zhao, who is border-line tanky DPS) can output decent damage, they are not tanky DPS.
4. Who is a tanky DPS?
Again, in alphabetical order, Dr. Mundo, Garen, Mordekaiser, Nasus, Olaf, Udyr. Other's that I'm not quite sure of: Vladimir and Sion.
The only way that these guys have of initiating is just wading in, swinging their axe or mace. Their plan is to get in and **** up the opponent as much as possible for as long as possible. Mundo can take a hell of a lot of damage without dying, doing AoE damage the entire time and flinging cleavers. Garen will spin to win, Olaf does more damage as you hurt him, Udyr will proc phoenix more often and stun as much as possible, Nasus will prance around like the sexy ulting mother****er that he is and Morde is at his best if he has survived long enough to get himself your carries ghost as his *****, which he will then use to murder your entire team. Vladimir wants to ult and hurt you during his ult, and he gains damage from survivability (although I'm not entirely sure if I wouldn't just count him as a mage).
They differ from tanks in one key aspect; they DON'T initiate team fights, although they might gank well. They DON'T protect your carries well, with their taunts, displacements or mad CC (Udyr being the most CCed up of them all); at best they can protect your carries by doing AoE damage all around them so that no one wants to go near.
So... next time you need a tank to initiate or to protect that squishy Ashe, Kog'Maw or Miss Fortune, don't take a tanky DPS because they are not tanks.
tl;dr: Morde is not a tank. Also, the poll is included because I didn't have enough troll in my post.
Why the downvotes?