It is very true that the numbers may not 100% accurate however it depends very much where you need to use the counts from all tables in a database and for what purpose. Microsoft recommends to run DBCC UPDATEUSAGE for a Databases upgraded from SQL Server 2000 that may contain invalid counts but after that they should be pretty much accurate - see "best practice" under http://msdn.microsoft.com/en-us/library/ms188414.aspx.

Besides that I believe that when you run such a query against a live database you do not expect 100% accurate counts because if you think: by the time ALL the counts are completed the numbers in some of the tables will be off due to DELETEs and INSERTs that are inevitable in a database. Hope this clarifies my suggestion to use sp_spaceused.