Its true that if we all geared up to land the biggest fish you (could) hook in a given water then chances are we would all be using ridiculously heavy tackle, there has to be an element of compromise involved.I want to use gear appropriate for the species and sizes of fish I'm targeting. I don't worry about the 30lb+ carp that could take the bait intended for roach.
In terms of the orginal question I have been pretty happy using 3.2 Drennan float fish for Chub trotting. The FF lines either side of it - 2.6 I would feel a bit undergunned and 4.4 I think would be a bit crude. 3.2 seems to be a good compromise between strength & finesse, for me anyway and has even coped with a number of unexpected river Carp that gatecrashed swims. Not sure how it compares diameter wise with some of the other lines out there. As for pre-streached lines I see them as being hooklinks not mainline.
Matching lines up with the right rod makes a big difference too imo...heavy stiff rod and light line wont be a good combo for example.