The knights think Arthur takes Merlin along because...
9
votes
sooner or later, Merlin will surely die; Arthur wants a new servant
his sword needs immediate polishing after action (what? get your minds out of the gutter, metal can rust!)
Merlin is actually Uther's spy to make sure his son does as he's told, and Arthur has turned him into a double agent
Merlin makes excellent bait - that's MONSTER bait, nothing else, tsk
living with Gaius, Merlin surely has picked up some medical skills if someone gets wounded
that boy just needs some sunshine and exercise, being all bones and white skin
if you leave Merlin in Camelot unsupervised, it might not all be standing when Arthur returns
he doesn't want to show favouritism among the knights, so his manservant comes in handy (stop that line of thought)
Merlin is really a powerful sorceror, and the knights have noticed, even if Arthur hasn't
of a reason I will tell you in the comments