To be honest, most riflemen I see don't use their nades very much at all. What I'm describing here, though, is that (I think) a smaller percentage would be needed over all. Say if you had a squad of troopers, you'd now have a rifleman that was just as effective as a engy at taking down buildings, with the added bonus of them being more deadly. You wouldn't change all the engies into riflemen, but I think there'd be a greater percentage of them around. At the moment, what limits riflemen the most are buildings and their inability to destroy them, with seismics, that's moderately mitigated.