Change the title to: "should casters under level 5 just be gishes?" because that's what I feel this is. The argument (pardon if I read it wrong) is to dip some martial class to stay relevant until casters start to pull ahead, to which you either become a competent gish or lag behind other casters 1-2 CL.

Note that divine casters likely face no such problem -- Medium BaB and d8 HP is just enough to let them survive lower levels.