Index Distribution Statistics And Show Query Plan

I have an index that shows distribution statistics of 98.20%, which is very poor. I set show query plan and show statis I/O on. This table has 1113675 rows of data.

*************
select orderID, custId, intertcsi from tblorders
where intertcsi = '2815'

STEP 1
The type of query is SELECT
FROM TABLE
tblorders
Nested iteration
Index : indxInterTCSI
orderID custId intertcsi
----------- ----------- ---------
1015245 1011313 2815
2556392 2556392 2815
....


Table: tblOrders scan count 1, logical reads: 104, physical reads: 58, read ahead reads: 0
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Then I use the same select statement to force a table scan:

select orderID, custId, intertcsi from tblorders (index=0)
where intertcsi = '2815'

STEP 1
The type of query is SELECT
FROM TABLE
tblorders
Nested iteration
Table Scan
orderID custId intertcsi
----------- ----------- ---------
60472 61084 2815
102184 102333 2815
...
Table: tblOrders scan count 1, logical reads: 110795, physical reads: 6891, read ahead reads: 103980

When the index is not provided, the logical reads and physical reads increased dramatically. Does this tell me that I should keep that index though it is a poor selection? Is that because a huge table like this make the optimizer use the index. The query without using index takes longer time to run.
Any idea or comment would be very appreciated.