Good debate on this touching in other threads, so I thought I'd take it here where it's better placed.
The latest idea as raised by others is that, with Placidus, at a certain point above and below the equator, we all have 'locked' and 'hidden' planets, which is something which doesn't occur to equator-children, which is totally unreasonable. I have to say the research for why Placidus became popular doesn't look great for it, neither.
I'll say this; just like A50 says she's found a stellium in her chart which is totally not her in Placidus, Whole Sign puts my Mars in the 12th, which according to most modern sources I've found, would have you believe one is a rabid dog, ready to blow like a volanoe, and hating people behind closed doors in the meantime.
So Placidus I struggle with because of interceptions (unexplained, and every Jack and Jill is adding their own twist on it now, it's like new art, what do YOU see???) it's historical back-up is debilitating, and while I respect Whole Sign's origins, we come to the 'we can't ditch every development in astrology SINCE the ancients vs let's ditch the modern lost ways and return to the old teachings' argument, and Whole Sign 12 Mars doesn't ring a bell, a whilstle or click with me, and newer findings suggest Equal may have been an improvement on Whole Sign by (don't shoot, don't shooot, Ptolemy??) rather than the 'simplistic' view some take on it, giving it in fact a hidden historical boost, and as Bob said, we can't pick the house systems which we like...
\rant