Required? No, I can't imagine any realistic situation you'd really
need the bigger gun. However, I believe it would be useful in many of the duties the ship is likely to perform.
Having the big gun means you can deliver damage at a good range at low cost. Using missiles (or even ER shells) gives you much less bang for your buck.
For supporting allied operations, however, it would be nice to have some "cheap" alternatives to offer. With a bigger gun, the ships can be tasked with costal bombardment, rather than blowing through millions of dollar's worth of missiles. They can offer something useful to a combined force, simply because they don't need to conserve every shot.
Are they okay in the corrosive environment? I thought most equipment got chewed up pretty fast if it wasn't specifically designed for it.
Well, if it would be expensive to redesign the Hobarts to take the gun even at an early stage, that's the major rationale for having it blown out of the water...