Something like this could do the trick and you can convert it into an inline table valued function which would be as fast as the normal query.
DECLARE @Start date = '20120918',
@End date = '20140918';
WITH E1(N) AS (
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
), --10E+1 or 10 rows
E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front
-- for both a performance gain and prevention of accidental "overruns"
SELECT TOP (DATEDIFF(DD, @Start, @End) + 1)
DATEADD( DD, ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) - 1, @Start)
FROM E4
)
SELECT N
FROM cteTally l
WHERE DATENAME(DW, N) NOT IN( 'Saturday', 'Sunday');