Will this mace be replaceable come ICC? Is it worth finishing the second half of one mace and possibly building a third one for ICC or would it be a complete waste of possible progression time?
Its funny, for a druid I really don't think it will be replaced in ICC. With Glyph of Rapid Rejuvenation - Item - World of Warcraft and ~90 haste below the haste cap for non-LB hots, my amount of healing possible during a Rejuv-blanketing situation has increased, thus the amount of healing I can do during the blessing proc (and the amount of shielding I can do, obviously) has also increased. If you take my recent ICC10 parse World of Logs - Real Time Raid Analysis for example (tab over to my healing by spell) it attributed for over 20% of my effective healing done throughout the instance. Now, there are tons of opportunities to blanket the raid with HoTs, so this may be inflated from a "normal" raid scenario, but you can also look at this recent ToC25 normal parse (I think there's Sarth in there too >.>) World of Logs - Real Time Raid Analysis, its still attributing for over 10% of my effective healing.
Now, you can make any argument you wish as to whether or not shielding counts or doesn't count as "healing done," but if you count it as effective healing, that would mean I would need to find a weapon that would increase my healing by 10% to overcome the value of the blessing. I really don't see this happening in ICC.