I've seen a lot of people explaining why Black Cleaver and Last Whisper is a bad choice against highly armoured opponents. The Flat ArPen comes before the LW's %ArPen and this hinders the % a lot. I agree with this to an extent.

You could counter argue that because ArPen won't go into the negatives (So a champion with 10 armour wouldn't have -5 armour if attacked a single time by a champion with a Black Cleaver) that you could take the Cleaver for killing squishy champions and the Whisper to add a multiplicative % ArPen bonus for bringing down heavier enemies.

At any rate people seem set on believing that both of these items in the same build are just bad news.

If this is the case why do the same people continue to build Sorcs Shoes with 30Mpen from their runes and a Void Staff? Genuine question, I'm perplexed. surely the 50Mpen and the 40% Mpen Void gives you is the same thing as the Black Cleaver and LW.

You can't even compare those:
While Last Whisper and Void Staff are rather comparable (~2k gold, flad ad/ap + mpen/arpen), Sorc's and Black Cleaver aren't comparable at all.
Flat arpen/mpen is strongest early game when people have low arpen/mpen. You can get Sorc's early game without delaying your build because you'd get boots anyway. You can't get Black Cleaver early game because it costs ~3k gold and thus it's no early game item but replaces one of your bigger items (usually IE/Bloodthirster).

Without any other items, Black Cleaver is usually the strongest physical damage per gold item but its problem is that it just doesn't scale well with other items. IE/BT/PD have great synergy and benefit greatly from getting a LW while BC doesn't synergize really well with anything and actually gets worse with LW.

Depends how high your opponents' magic resist is to be honest.

I can't pretend to know too much of what I am saying as I am relatively new to this game and trying out new champs at the moment but I am more than confident with Kassadin.
I find that if the opposing team are stupid enough to go without any serious magic resist I go for Ionian boots + void staff as the staff stacks with rabidons and cooldown allows me to riftwalk so much better.
But of course like most things in this game it's situational I guess.

My main preference is to go with the sorc shoes as most people I play now are smart enough to understand mpen.

What you buy early game doesn't make a lick of difference if it carries the same effects over into mid/late when you would legitimately have all of these items. You would still be ruining the efficiency of the Void Staff just as you would the LW.

It's exactly the same with flat magic pen. You get sorc boots for the early / midgame and void staff for the lategame. The thing is Black Cleaver is too much of an investment for that, it's not cheap like boots.

Please read my post again.
Oh, and VS/LW are always effective late game against competent players no matter how much flat mpen/arpen you have. It's more that they diminish the value of your flat mpen/arpen.

And even if you insisted on not having both at the same time, you could still sell your Sorc's and buy Ionian/Merc once you get Void and you'd only lose like 350g while having gained more damage for ~20-30 minutes.
Selling Black Cleaver usually is no option because you'd lose 1k gold for ~10 minutes of it being slightly better than its alternatives at most.

My view doesn't change. What was discussed in these threads before was that getting both items ruins the effiency of the % ArPen item. Not about how it affects your build and how it isn't an early game item.

My argument is that it's exactly the same for the sorcs, mpen runes and void staff. I understand you would buy sorcs first because you need that bit of early game mpen. However, were I to compare 2 Champions.

1 with AD runes, a Black Cleaver and a Last Whisper
1 with Mpen runes, Sorcs Shoes and a Void Staff.

Would the Mpen runes and sorcs shoes not strip the Void Staff of its efficiency late game in the same way that the BC does to the Whisper?

On top of that why not sell your Sorcs Shoes late game for another pair of boots? Mercs Treads for example?

No one has ever explained that before. (Atleast not that I've seen)

And on top of all of this I see some guides which tell me as Lategame Items to buy Void Staff for the Mpen and also an Abyssal Sceptre. Does this not also reduce the penetration of the void staff given that reduction comes before penetration?

You wanted to know why it's considered bad to go BC + LW but it's usual to go Sorc + VS at some time. It's not because it synergizes any better but because of the reasons I stated.

I wanted to know why it was constantly announced that BC and LW were bad combos because they reduced efficiency of the % arpen and yet the sorcs,runes and voidstaff combo was never given the same thought yet they do the same thing, regardless of build hinderance or early game buys.

My question still stands.

Does building Sorcs Shoes and a Void Staff whilst also having Mpen runes all at the same time reduce the efficiency of the Void Staff and in that case would I be better selling my boots and buying Lucids?

BC + LW isn't going to make u do any dmg to tanks, you simply NEED an infinity edge or you'll be doing **** damage even with last whisper.
It doesn't matter if the values are good, you are either giving up crit and dmg (from IE) or lifesteal and damage (from BT), why would you ever even bother with BC.