Marriage and other documents in Guatemala have information regarding grandparents, so it would be useful to register this information directly in the same documentation.
Presumably you're talking about adding indexing fields for the grandparents.
Indexing is always a tradeoff between speed and completeness. Generally, projects are set up to achieve the best level of usefulness with the least amount of work: just enough information for people to find the records they need, but not so much that the index will never be completed. When you consider your question from this angle, you can see why the grandparents aren't included in indexes: what are the chances that anyone would search for someone based on his or her grandparents?
Keep in mind that the index is not the data. It's just a finding aid for the data.