Step one, check the errata statistics, it has optional rules for retooling weapon damage. I personally like them, but they're up to you and your group.
Not sure where you read pistols require Two Weapon Wielder to be used in melee. They don't. Now yes, to use the iconic pistol and sword combo to its fullest effect does indeed require the Two Weapon Wielder talent (that is to say, the ability to make a melee attack and fire the pistol as a full round action, possibly benefiting from swift/lightning attack). But if your character is more ranged oriented, and an enemy has engaged you in melee, you are probably better served drawing your bolt pistol than trying to either back away and fire a basic weapon or using a sub-par melee ability. Without TWW, certainly, a bolter and chainsword approach is also quite effective.
The ability to use a pistol in melee is about its only draw over a basic weapon. Also, depending on your GM and strict readings of the rules, TWW(ranged) can't really be done with two basic weapons (if a player tried to dual wield storm bolters at my table, I'd be introducing their face to the casing for my Deathwatch CE. And no jury would convict me), but pistols do work. Dual shot with inferno pistols comes to mind with this. If the pistols have semi/full auto they can fire that as well.
The smaller size of a pistol weapon means less GM frustration with players trying to carry 27 different basic weapons on their person. Yes, Astartes carry capacity is amazing. The problem is one of bulk, not weight. Pistols are a great ranged alternative if the player is already bogged down carrying a power sword, a flamer, a combat knife, and a chainsword.
The only real downsides a pistol has in this system though is the effective range, rate of fire, and ammo capacity. Damage/pen are the same. Range tends not to be an issue, as the intent is to use them at short range. Rate of fire isn't that terrible a loss, as to make full use of that high a semi-automatic (this is assuming you're using errata weapon stats) requires a pretty high base BS and a solid roll. Capacity certainly is an issue, but either weapon can still fire its max RoF for 7 turns before needing a reload.
This is pretty much a roundabout way of saying sidearms are useful for melee.