the other way you can look at this is that if you know your limits and set yourself up to go just to the brink of failure and barely finish your last rep then you might be at less risk for injuring yourself...and you still trained damn hard.
As for drop sets...I'd rather do a rest-paused set!
Or go to failure on machines (hammer, smith, etc.) then ask someone for a spot on other stuff. I train alone and just ask for a spot if I need it.