I'm playing a Monk/Rogue, and up until recently played a Fighter/Rogue in another campaign.
With lower BAB, I don't hit as much as the pure Fighter types. The Fighter/Rogue was also a two-weapon fighter, which further reduces To Hit in favor of more attacks. Two weapon fighting also tends to use smaller weapons that do less damage.
In both games, there is a high strength, Greatsword wielding Fighter who does WAY more damage than me - consistently. Oh sure, when I get my sneak damage, I get up there, but those guys do it regularly.
Put it this way (for lower levels): Light weapon (d6) + (maybe) 1 or 2d6 sneak and maybe a point or two of strength damage, versus 2d6 for a Greatsword, plus 4 or 6 strength damage ON EVERY HIT, with a much better potential to hit. Which would you rather face?
And again, the Rogue type only gets that bonus damage when flanking, or catching them flat footed, or in other special circumstances; and doesn't get it at all against a lot of potential opponents. Heck, my Fighter/Rogue was in Ravenloft! You know how many opponents he actually got his sneak attack against???