If you run rebuild and update stats when nobody works with DB then it will be like you said, but if you have 24/7 system who will guarantee that nobody update at least one record between rebuild and sp_updatestats operations? And if such update happens then sp_updatestats will update your statistics with sample rate because based on rowmodctr that statistics requires updating. You can test it yourself:

CREATE DATABASE UpdateStatsTest

GO

USE UpdateStatsTest

CREATE TABLE dbo.T (ID int NOT NULL)

ALTER TABLE dbo.T ADD CONSTRAINT PK_T PRIMARY KEY CLUSTERED (ID )

INSERT dbo.T(ID) VALUES(1)

GO

ALTER INDEX [PK_T] ON [dbo].[T] REBUILD

INSERT dbo.T(ID) VALUES(2) -- Insert one record between rebuild and updatestats

EXEC sp_updatestats

Here is result, stats has been updated:

Updating [dbo].[T]

[PK_T] has been updated...

1 index(es)/statistic(s) have been updated, 0 did not require update.