I don't get what's the point of going to the gym and half ass it? If you know you got one or two more reps in you why not do them?
Because it doesn't give you better results, at least in my experience.
Especially as a natural you have to budget carefully with the recovery ability of your body. Going to failure all the time is enourmous stress for the CNS and the regeneration of the muscle takes much much longer, as a natural you might even break down more muscle tissue than you'll be able to rebuilt afterwards.
Of course when I was a young, hungry Padawan I too punched every single set to total failure, but my devise nowadays is also "Train hard BUT SMART". I hardly ever go to failure nowadays, most sets just that I can get the last positive rep on my own and that's it, no spotter, no forced reps, no partials or swinging.
I feel much fitter and better nowadays (when I trained to failure all the time I was tired often all day long and had sore muscles in some groups for 4-5 days) and still get almost better results.
A natural imo should be careful with total failure, maybe even avoid it completely. But like with anything in BB, I guess everyone has to make his own experiences.