Let's just agree that Luzan alive adds more value to Blackbeard than dead. Kuzan is a former admiral who became known after MF for losing a fight and leaving the organization. Engaging in a fight with Kuzan would create a breach that other Emperors could take advantage of, especially Shanks, who has literally been looking for Blackbeard. On the other hand, if Blackbeard were to duel with Luzan and defeat him, all he would have is his fruit, and only that. It may add some credibility to the feat, but it wouldn't be anything alarming. There wouldn't be anyone of Luzan's caliber to consume his fruit and be as effective as he is in his tasks, so killing him would be a waste.
If it were at least Kaido instead of Luzan, it would make total sense. This is because the BB Pirates would be able to disrupt the Beast Pirates, possibly with the hypothetical fall of Kaido, they would obtain Wano as their stronghold and the resources that Kaido has built up over decades, in addition to Kaido's ROAD. Do you understand that this is simply on another level?
That's right, Luzan isn't even actively competing with BB. LOL