Nice naivity about one char only. I agree that you can play only with one char, if you dont PvP, but if you PvP, you need:
1) main battle characters: long range and short range (because big gun burster cant to almost anytning in Klamath and your SGsniper is dead immediately in Redding .. switching weapons does not help, cause the effectivity is bad)
2) if you pvp, you need somewhere to get weapons, ammo, armor and drugs, so you need at least one from those chars:
weapon crafter, armor crafter, drug crafter, looter, trader, thief
3) also you can use special chars like:
lockpicker, mutant leader, demoman, spy, transporter
So if you 3 or 4 chars are minimal for pvp and of course, you have to use services of your mates (mostly drug crafters). Also there are people, who likes testing new builds, and they can have even 10 main battle characters.
I seriously hope this could help devs to implement new changes to skill system. Of course, they will not eliminate alternative main battle characters, but at least reducing the neccesity in MBT and weapon crafter is good way how to eliminate secondary unnecesary alts.