That is absolutely incorrect, Caine. In fact the last point of haste is by far the most important and represents a massive gain in DPS. The last point of haste needed will always free up 2 GCDs per cast of DP. This means the DoT is doing the same DPS, but you have now got an additional 2 seconds before having to recast the DoT. By reaching breakpoints you get more Mind Flays in your rotation while losing no DPS from your DoTs. When you increase Haste from say 1269 to 1458 you will in fact increase the DPS value of DP, but the damage it deals per cast is unchanged. When you go from 1458 to 1459 you gain a tiny DPS increase on the spell (<.015%), but you increase the damage per cast by 8.3% and get an additional Mind Flay in your rotation for each cast of DP.
Do not underestimate the value of breakpoints.