- You don't have to be right on time when the next deal comes out.
- You don't have to worry if it will be unavailable when you sign in later.
- It is very unlikely, but you may be able to get something cheaper then it's original cost.
- You will probably end up paying more for the tank then what is it actually valued at.
- If WG went with fixed prices instead, you will most definitely pay less then what you bid for most auctions in the BM.
And that is why I believe WG did more auctions instead of buy it right now prices for the BM, because they know you will be paying way more then you should have.
(I might be missing some Pros and cons, so feel free to list others in the comments.)