When it comes to witnesses most couples choose their chief bridesmaid and best man, usually because this is the way its taditionally done and often because they haven't been given any other options.
But let's face it, the best man and bridesmaid already have a great job to do on the day and making other people your witnesses is an oportunity to involve them in an important part of the ceremony. If a groom has had a hard time choosing between two of his best friends to be best man, he can make the other one a witness. I've also had a few weddings where both mums have been witnesses and this is especially lovely because, unlike men, women don't have such a high profile role in ceremonies as they get older.
At one wedding I saw a lovely choice of witness. The bride's parents had split up when she was very young and she was close to both her father and her step father. Her father walked her down the aisle but her step father was a witness, which was a touching way of making him feel important on the day. I've also had a couple's (grown up) children be witnesses, aunties, siblings and friends who weren't as close to the bride and groom as their best man and bridesmaid but who were the people who introduced them to each other.
In a humanist ceremony there is the opportunity for the celebrant to introduce the witnesses and say why they have been asked by the couple to be part of the ceremony.
Making someone a witness is a good way of making a special person in your lives feel important, but if you choose to have your best man and bridesmaid be your witness, that's fine too!