Well I have had this theory for a long time and I was checking the topics and I found the OP ones kinda dead. Even though I do not like OP last years I think there are a lot of things for us to discuss so I decided to share this theory (of non importance since WB is long gone).
Well, firstly Oda was really clever in introducing WB as an old guy who had lost a great amount of his power due to his age, because in other animes this isn't the case....(avoid spoiler if you do not follow manga of Bleach)
- Spoiler:
In Bleach Yamamoto is introduced as the strongest Shinigami born in the last 1000 years and then Quincy leader cuts him in half like nothing.
So WB probably wasn't able to use his Haki, he had Haki in his prime (surely he was the strongest man in the world afterall) but he couldn't use because of his age. I base my opinion on these facts:
1. When WB fought Aokiji he was breaking his Ice Spears and Aokiji's body (which was turned into ice) using his Fruit powers which work like power waves that travel through air and objects. Also when he stabbed Aokiji he did not hurt him at all, on the contrary his spear was being frozen but Joze saved his Dad.
2. When WB fought Akainu, well not really fought more likely destroying Akainu I think he was able to hurt him because of the nature of his power (those power waves that can pass through the body) since Akainu wasn't able to turn his whole body into lava like Aokiji did using ice.
3. WB obviously hurt BB but as we all know BB's DF is different from the other logias. It does not give him the same defence as the others (transforming his body parts toa void damage) but on the contrary Yami Yami No Mi increases the amount of damage that its owner takes (It was stated by BB in his fight versus Ace).
4. Well this isn't that important but WB did not use his Haki to stop Ace's execution like Luffy did without realising it but he could also use his powers I think like Crocoboy did. (Althought WB's powers would hit everyone including Ace not only the guards).
5. When the BB pirates were shooting him he could have used his Haki to block the bulllets. Okay someone told me (Crookz or Nikos150) that the bullets could have Haki from the BB pirates but I do not think that the first BB's crewmates (Laffite, Burgess, D Auge, and the doctor) had Haki at this point of the anime. The Ex-Level6 prisoners and BB himself probably had Haki.
The only doubt I have for my theory is the fight WB vs Akainu, I mean WB could have hit Sakazuki by using Haki which is the most possible scenario but since he never showed real signs of Haki I will bet on the nature of his Devil Fruit powers.
Also when WB clashed with Shanks the sky seemed to be split in two. This was clearly a sign of WB's Haki but still he was sick and this happened after a considerable time before the war (and WB was under doctor's care so he was probably in a better shape than the war). I Just remembered that, oh god how could I skip WB's first appearance.....
I know this theory does not have much of importance but I just wanted to share my thoughts. If I recall anything else I am gonna edit it. Feel free to share your opinions :D