Depends how many Elementals there are. Suffrens carry a LOT of troops. Even if we assume the Suffren's shuttles can only carry one platoon apiece and the rest are left behind, that's still 224 marine points, or 384 if using the published Comstar space marine platoons. That first number's equal to a full Trinary of standard Elementals with small lasers, while the second is equivalent to a Trinary of Space Elementals that have swapped out the lasers for machine guns, flamers, or any other burst-fire weapon. You'll note that this is more than the troops carried aboard a Carrack, Odyssey, or even a Liberator.
Don't forget that those troops can also be raiders, allowing you to attack targets groundside where armored units might be unavailable or impractical. A Suffren's a bit much for an infantry raid, but I'm certain the captain appreciates having an option for reacting to planetary situations in a way other than 'Glass it'.
As for the Suffren itself, the gun and armor layout scream fast attack ship to me. Line up a high-speed attack run on a station, or a fleet at anchor/on a known course, then come screaming in out of deep space at 0.1C. Anything that survives your nose guns gets the tailshot in the next microsecond, you're gone before enemy fighters have a chance to do real damage to you, and any shots that do hit you will come in on your nose or aft charts, where your armor is thickest.