I run a Space Marine bike army very often. In fact, it's sort of my specialty. Writing about the Biker Sergeant is less theory and more practice for me (yes, some of the previous Sergeant Series articles were based somewhat in theory).
The basic Biker Sergeant doesn't fill much of a unique role any more, not that he ever really did. You see, Bike Squads are defined not by their leader, but by their special weapon troopers. For the most part, the Sergeant is just along for the ride. In Fifth Edition, he came stock with Ld9, so keeping him around helped on Morale tests.
The first upgrade to look at for the Sixth Edition Biker Sergeant is Veteran status. Its most obvious benefit to a Bike Squad is Ld9 over Ld8. Just like an Assault Squad, a failed Morale test is incredibly dangerous for a Bike Squad in the early going. 3d6 fall back moves when in your own deployment zone can quite often see an entire squad off the board in the first turn.
Less useful is the +1 attack provided by Veteran status. Biker Sergeants don't really need more attacks.
Why don't Biker Sergeants need attacks? Because Bike Squads, no matter how you slice it, are NOT melee units. Ever.
Your Biker Sergeant has access to all of the same melee options as the rest of the sergeants in the book: power weapon, lightning claw, power fist, and thunder hammer. You know what they all do by now, and none of them are worth taking on a Biker Sergeant due to points costs. You can argue that a power sword is good insurance, or that a powerfist serves well against walkers. Bikes are nimble and fast enough to avoid these units and situations. This is doubly true when you use the White Scars Chapter Tactic in order to gain Hit and Run. You should not be stuck in a hopeless combat with a walker for a whole game any more.
One situation of note is the challenge. Biker Sergeants can save their entire squad from a big monster or face-crushing character by issuing or accepting a challenge. On the charge, you will ALWAYS get your Hammer of Wrath attack, because once the challenge is accepted, you move the two combatants into base contact, triggering the Hammer of Wrath attack! You get one free hit at S4 and I10. Against small fry like Guard sergeants and Tau Shas-whatevers, it can end the challenge before it really begins, and mitigates the need to take that insurance power sword or maul.
Bike Squads are designed for delivery of shooting systems like meltaguns, plasma, and grav guns. Biker Sergeants have access to pistols, combi bolters, and storm bolters.
I'll tell you early to skip the storm bolter. It serves no purpose on a Biker Sergeant whose bike already provides a twin-linked bolter. The one additional bolt shot at >12" range will rarely be of use.
Pistols are of greater utility on a speedy unit like Bikes than they are on Marine infantry units. A plasma or grav pistol can be maneuvered into firing range by a Biker Sergeant quite well. The trick is finding the points for that pistol when compared to other options. It's important to remember that a Biker Sergeant will never get bonus attacks for having a pistol, as he lacks the capacity to have both a pistol and a close combat weapon.
Biker Sergeants can select from any of the four combi bolter types: plasma, flamer, melta, and grav. They're all useful, and my recommendation is to select the one that matches the special weapon in the squad. taking two plasma guns? Arm the sergeant with a combi-plasma. Grav guns? Combi-grav. It's a pretty simple formula.
Biker Sergeants can also access meltabombs. If you have the points, take them. You won't regret it.
I personally run all of my Biker Sergeants with a combi of some sort (though my plasma squad's sergeant carries a plasma pistol, because I like the look on folks' faces when I say I paid for a plasma pistol). I leave the melee upgrades to the Command Squad. You cannot buy both a shooting and a melee upgrade for Biker Sergeants due to his base wargear only including a bolt pistol. You can't do the "best of both worlds" stuff with this guy, like you can with a Tactical Squad Sergeant. Stick to shooting and grenades/HoW hits for bikes.