No, you can book with point and don't need 50,000 points to do so. If you sign into your account online, and are on the site, go thru the motions of booking a flights. Select "points" instead of "$", and it will tell you how much each leg of the flight costs in points. As long as you have enough points for the ticket, you are good to go.
So lets say for instance a flight from LAS to DEN, round trip, was 23,500 points. You could book that using points from your pool of 43,000 points, and you'd have a balance of 19,500 points to use later.
You can book flights for others using your points as well, I just booked my husbands flight to MCO with points from my account, since my account had more than his.
And say you have 43,000 points, and the trip you want to book is 50,000 points - well you can book one leg of your flight with points (as in a one way booking), and then book another one way and pay cash for the second leg of your flight.
You can't use your points for the taxes or government fees...I forget the exact terminology, but it usually ends up being $5-$10 per r/t ticket I have to pay out of pocket for.
I find Southwest to be very accommodating and the points very easy to use.