Assuming the server seed commitment is known before placing a bet, the server can't alter the outcome in any way. Is is the idea that a server could predict betting patterns for an individual user (and shuffle cards / pick numbers accordingly)? That's the only thing I could think of which a client seed does prevent.
Yes, gamblers tend to be very predictable in their bets. If the gambler has the opportunity to add unique random data to the calculation of the result, the casino cannot predict the outcome of the bet ahead of time.