Alphadog, the point papa is making is that the output low level voltage depends on the amount of current the output is sinking. The more current the output is sinking the higher the output low level voltage will be.
Likewise, the output high level voltage depends on the amount of current the output is sourcing. The more current being sourced, the lower the output high level voltage will be.
I don't agree that the use of a diode to raise the threshold for turn-on is "completely useless". If the transistor is driving a large load (say a relay), a few milliamps of base current could be necessary to drive the transistor into saturation to energize the load and a base diode does indeed reach its full forward voltage only when a sufficient driving voltage (and thus base current) is reached. It may not necessarily be the best way to do it from a given standpoint, but it can be effective, depending on the load and the effect of small amounts of base current.
BTW, making a statement with the phrase "completely useless" without explaination from someone seeking comment and assistance, is irresponsible and rude -- especially from a "super moderator". It reflects poorly on the forum and its moderators. Maybe the forum's real purpose isn't what it is purported to be?