This is Tech's way of keeping safe from Viruses, but not everyone can have a 6 different scanners and still have a peaceful computer. 
Sure. But I don't keep me safe with them. My protection is avast, the only resident.
The others are for test purposes, backup scanner, learning, features comparison to make suggestions to avast!, etc.
For sure, don't have all of them for security reasons.
As backup scanner, an order for me would be:
1. ClamWin
2. AntiVir
3. BitDefender
4. AVG
5. F-Prot DOS
Taking in account the hability of update and the detection (at least for me).