I have to agree that using multiple planets in a single sector to get faster training is NOT a cheat/bug/flaw; I think it's a neat idea. I'm sorry I didn't think of trying it myself.
I'd also like to mention that using training facilities against the AI isn't necessarily "unsporting". At least one of the stock AIs (and, I suspect, many of the TDM AI's) use racial points to improve offense and/or defense. I usually put points into construction, repair and research (not to mention Temporal Knowledge & advanced propulsion), so I've ended up on the receiving end of what amounts to a better-trained fleet. It's not pretty. In fact, it can be downright upsetting when you only hit with 10% of your shots while the enemy AI completely destroys your fleet. So, when I meet one of the offense/defense bonus races I'll build training facilities just to even the odds (or tip the scales in my favor, of course
).