Power from the biceps is crucial for picking balls up off the court.
Seriously tho', the biceps are used for bending the arm at the elbow, but are not used directly
that often for generating power -- they are used indirectly, bending the arm, so that other triceps and other muscles can apply power.
If you use a lot of supination
(opposite of pronation) of your BH strokes, the biceps might be a factor in power production. (This is probably more common in badminton than it is in tennis). It should be noted that the biceps are involved in supination primarily when the arm is bent -- when the arm is straight, the role of the biceps is considerably less. You probably use some biceps power when supinating on a BH overhead (smash).
Even if you do not believe that you need much power from the biceps for your strokes, it is still a good idea to work on them if you plan to develop your triceps. The biceps & triceps are complementary muscle groups -- if you work on one group, you should also work the other to prevent muscle imbalance
. On the other hand, you probably don't need to work on the biceps quite as much as you work on the triceps.