Mostly, the issue is one of preference, perception, and circumstance. If you have a fighter who views himself as a holy knight, then Succor is entirely appropriate for the "laying on of hands" thing, as compared to learning spells to a level that it'll work through his heavy armor.
Plus, look at how many people are in the party, and how many fights they have in a given play session. If you end up in a small group (3 or 4 people), you might need the extra healing, especially if the cleric can't make it to a given session. If it's a big group (let's say 7 or 8, or even more!), each character can get 1 shot of healing, or 2 at the most. That sometimes isn't even enough to heal up after a single fight (especially if you roll poorly)!
So yes, at high levels, Greater Succor becomes a decent supplement to a cleric (or vivamancer), but it's not overpowering. I consider it on par with a rogue with the minor healing spell. Yes, it's nice to have, but it's not going to replace the main healer by a long shot!
