It will stand, mostly because its a good idea to allow players who are geeked out about grey knights, but since I was the big pusher for the barge back when we were working on it, let me explain my mentality.
1. There is a new grey knights codex, not giving them something big and glittery would be extremely short sighted and a quick way to turn people away from the fleet.
2. Grey knights are the ones sent in to eradicate other space marine chapters who have been determined to be excommunicate. These chapters usually have fortress monistary's, two battle barges, planetside bases and fleets of strike cruisers and escorts. Not giving them something that can stand up to battleship level fire is just contrary to their purpose.
3. They are still a space marine chapter, and the only one fielded on Titan. The idea that they would not have a barge with Mars so close by baffles me. When you have the best equipment and tech priests at your fingertips, you never, ever say "no no, we want something smaller."
4. When dealing with large demonic threats, like demon worlds or demonship fleets, strike cruisers and escorts alone would be easily outmatched. Without access to battleship level defense or attack, they would not be able to do their job.
5. Its a damn pretty ship! I just wish we had the psy-cannon on the prow.