It's best for extra healing on targets you aren't chain casting on. It's not really designed to drastically increase tank healing throughput. It's designed for raid healing/light tank healing throughput. It'll still do -something- when you're busy with only tanks, just not it's full effect. Makes sense to me.
It will still tick every second regardless of getting refreshed; this is how dots like Shadow Word: Pain and Corruption roll as well. There isn't any healing lost which makes it equally powerful on the raid or on the tank.
It refreshes and stacks but not infinitely, the stacking only occurs for the 6 seconds after each spell, at which point the spell cast 6 seconds ago's ticks will be deducted from the current ticking.
As Lysdexic said, you will still get the full amount of ticks, irrespective of refreshing.
Echo of Light and Ignite from Fire Mages have the exact same mechanic. The size of the HoT refreshes dynamically per tick. Basically the HoT is not 'stacking' in that you don't see an increase in stack count. Instead it's usually called 'rolling'. What this means is that each time you refresh it the value is saved. So say you heal 3 times for 10k. You'll have 3 'stacks' of a 1k HoT (with 10% mastery) each of which lasts 6 seconds from the time it is first applied, but is then merged into 1 buff. So you'll get ticks like this: 100, 100, 200, 200, 300, 300, 200, 200, 100, 100. You get ticks for 100 when the first heal is the only one up, 200 when two are up at once and 300 when all are up at once.
So in this manner it 'refreshes', but it's really Blizzards backdoor method to allow it to stack without creating excessive combat log flooding.