Amazingly, I've based my forms on queries for years and they're updatable! The fact of the matter is, most experienced developers will tell you not to base a form on a table, even if only one table is invovled, but rather a simple query based on the table! It all depends on the type of query, with most simple Select queries being updatable.
Here's a small article from Allen Browne explaining why some queries are read-only.