Aboslutely! A good manager can be a valuable asset with the right combination. It can get a good worker/bad talker over with the crowd. Case in point then: the Midnight Express. Dennis Condrey and Bobby Eaton were both great wrestlers, but neither was considered a good promo. Having Jim Cornette do their promos and running around ringside with his tennis racket made them one of the most hated tag teams ever. Now: look at Dolph Ziggler. Dolph has a lot going for him, but having Vickie with him actually gets people caring about him.
Even if a guy can talk, the right manager/valet can add to his appeal. Randy Savage was a good enough promo that he didn't need Elizabeth at ringside, but the combination worked big time. Bobby Heenan often managed guys who didn't really need a manager (Rick Rude, Nick Bockwinkel, ...), but he added to what they had and made them even more hated.
There are a lot of guys who could have done better if they had had the right manager. Shelton Benjamin was a fantastic wrestler, but he wasn't a good promo - and Momma Benjamin was NOT the right manager to get him over. Besides Vickie, William Regal has shown that he can be a great manager and can be one again if WWE chose to use him that way. Rosa Mendes could head up a Latino/Hispanic stable. Half the Divas can't wrestle for squat, but accompanying the right guy to the ring could help them be more than just a useless piece of eye candy.