Maybe it won't come to this, but I've seen many a mock that have both of the top OLBs, WRs, and CBs off the board. That puts us in a position that either requires a trade-down, a reach (I'm pretty sure most of us aren't aboard the Aldon Smith wagon), or taking a BPA that doesn't really fit a need. Hear me out as I explain why J.J. Watt is a viable option.
I don't think I need to argue ability or draft status; both are considered top-15 picks right now, and natural 5-tech DEs don't exactly grow on trees. The issue, then, is finding a place for for them on a team that already has two highly-paid players who could play the position. Antonio Smith already has 3-4 experience, and doesn't project as an OLB, so we'll leave him be. Mario, on the other hand, offers versatility. Despite his frame and strength, he has never played with the physicality of a five-tech DE. That's not to say he couldn't, but he has always favored his speed rush to inside strength-based moves.
I've heard Phillips' Bruce Smith comparisons. There's some merit to that; I think that if Mario buys in, he can have greater effect on this defense from the DE position. But let's keep in mind, as great as Mario is when he's healthy, he's fairly pedestrian once those nagging injuries set in. Placing him in the trenches against constant double teams isn't the greatest way to ensure he stays healthy. And while there's no certainty that he'll be effective standing up at WOLB, there's going to be a learning curve taking on new DE responsibilities as well.
Lastly, (and I'd urge you all not to write this off), there's the issue of Mario's looming free agent status. As a 4-3 DE, there aren't three better players in the league. You can imagine, therefore, that Mario's market value is going to be upwards of $10 million per year. I'm not sure he'll get Peppers money, but it could be closer to $13 or $14 million per year if he really wants to find the highest bidder. Is that at all worth it for a 3-4 DE? Looking at opportunity costs, I'd say no. By drafting J.J. Watt, you wind up with a potential BPA, allow Mario to play more of a pass-rushing role, and give your team flexibility when it comes to re-signing a player who might be out of your price range.