Ya I get that dude, your still WASTING GOLD TO GET IT. That gold is gone now and did nothing extreme for you. That armor shred is nice but you still wasted gold to get it.
Think of this, You got Tri, build Phage for BC, go back out on the rift, you now have a item that gives you what 15 AD and 200 HP for 1250 gold? Your missing out on like 400 gold worth of stats cause you already have the Phage passive.
1 AD = 35 gold so 15 AD = 525 gold
1 HP = 2 Gold so 200 HP = 400 gold
925 Gold, you spent 1250, so you wasted 325 gold. Doesn't seem like a lot, but when your going to your next item your 325 gold further away.
Phage itself is 82.7% gold efficent, without the Phage bonus its like 63%.
Tri is 97%
BC is 99% but building both moves one down to 80%ish.
So ya if your ok with losing out on overall gold efficient late game, by all means by em both, but you wont be as strong as you could of been. Also seeing how late game doesn't really come as often. I guess its all personal opinion, I'd never buy both. If they're tanky I'd get BC, if squishy i'd get Tri.