There are certainly benefits to him being a romanceable npc. For starters, we'll be more inclined to mix our party up more frequently without having to worry that not bringing him along might mean we miss something, we'll probably get away with doing a few minor things that he wouldn't agree with and not have to worry about sacrificing any of his approval...
... That's all I can think of right now for positives right now, lol.
That is so true! I always take my chosen LI with me everywhere, to the detriment of party composition.
I think we'd need to know more about how approval works, before calling that a positive.
It could be like Wynne and the Ashes quest, and everyone on your party is a blasted gossip.
Other positives...
He'll probably have very nice, unique armor. 
And this thread moves too fast!