I would agree with vanilla's comments. I am not an expert in conveyancing, but you should certainly speak to your solicitor to have this clarified.
An easement is a right to use land where you do not have title (own) the land without fear of trespass. For example, if you bought a house in the middle of a field, you're driveway to the open road may be by means of an easement over anothers land. This could also apply to companies like the ESB, whereby they can access land with out fear of prosecution for trespassing.
I assume this would apply to commercial properties - you do not know what deal the developer made with the land purchaser, there could be any sort of issue or dispute surrounding the land. By carrying out due diligence, I think your solicitor is quite right in requesting disclosure of documents that should be stanrad in conveyancing.
To answer your questions:
How important is a Grant of Easements? My solicitor is based in Cork where it is apparently seen as essential. Is it as important in Dublin?
I would think it is a country wide requirement. You should know up front all issues surrounding the property and any issues that may effect you long term, after the sale has closed.
Why would a builder and/or his solicitor refuse to furnish my solicitor with a Grant of Easements? Is there any advantage in doing this?
By not disclosing these, the builders solicitor could be hiding something, there could be restrictions for the use of the easement that may affect you, or there may be a dispute with the land owner over ehich the easement has been granted. Get this checked out with you solicitor.