I don't want to start a flame-war here, but if this is true then they are lousy DPS and / or you have a huge gear advantage. Try that with any competent DPS, and they'll rip aggro off you after 6 seconds.
This isn't entirely true. There is a common misconception on how Taunt works with regard to the threat table.
Tanks who open with taunt, should stop tanking. (Let's just say that upfront)
A taunt does exactly TWO things: (in PvE)
1- It guarantees that the taunted mob will not target anyone else for 6 seconds.
2- It puts the person who taunted, at the top of the current threat table @ 110%*** of the person with top threat.
*** No. 2 is VERY important... because threat operates at two different threat levels to pull aggro. (see below, it's not always 110%)
Melee & Ranged
Characters in "melee" range need to pull 110% of the threat of the person at the top of the table to pull aggro. So for the sake of argument, let's assume that the tank currently has 100,000 Threat, and the marauder attacking the same target has generated 105,000 Threat. Even tho, the marauder is technically at the top of the threat table, he will not pull aggro until he hits 110,000 Threat, because he needs 110% threat to pull aggro.
Characters from "range" need to pull 130% of the threat of the person at the top of the table to pull aggro. So for the sake of argument, let's assume that the tank currently has 100,000 Threat, and the sniper attacking the same target has generated 115,000 Threat. Even tho, the sniper is technically at the top of the threat table, (by more than 110% even) he will not pull aggro until he hits 130,000 Threat, because he needs 130% threat to pull aggro.
So what does that mean?
Well, to answer that question you have to define what "Melee" and "Range" are. And it's been determined (by people smarter than me who put in more time looking at this) that:
= 4m or LESS from the center of the target.
= MORE than 4m from the center of the target.
So, correlating back to the 2 things that Taunt does, and how important #2 is.... If you are tanking, and you are 3m from the boss and you TAUNT, you will get put at the top of the threat table + 10%.
If you step backwards to 4.1m and then TAUNT, you will get put at the top of the threat table +30%.
Basically at the end of the day, what that means is that if you back up a few steps before taunting (for threat, not mechanics) you SHOULD stay well ahead of the dps's threat because your taunts are generating 20% more threat then if you taunted from within "melee" range.
After cycling a few 130% taunts in a fight that doesn't have a threat-dump-mechanic or tank swapping? In theory, you SHOULD be so far ahead in the table that it would take incredibly stellar DPS to pull off you.
I'd have to dig around to find the proof for this, but it does work this way in-game.