I simply think it fits the role of siege/linebreaker better with forward mounted bombardment cannons, it feels more SM to me, not having to turn to bring your weapons to bear.
Also, it would make fluff sense, since the BC's are designed to rape defenses, as broadsides are more ship to ship fighting. And of course it would be cheaper.
Smotherman is just a guideline for a reason, and should only be very generously applied to marines, as it was built mainly for IN an Chaos. If you are paying 25 points for a weapons swap, not even an obvious advantage, you won't see the variants ever played in competative games. You are already getting into the points range of cruisers that the SC can't compete with.
Like I said, I really do agree with the Assault getting a 20 point increase, because it fills somewhat of a weakness in the fleet. Lets give the Linebreaker, or whatever we want to call it, a 10 point increase. It definitly won't be unbalanced if you compare the variants.