Let's actually get some sensible referential material into this debate.
"No wizardry, nor spell, nor dart,
No fang, nor venom devil’s art
Could brew had harmed him; for his weird
Was woven. Yet he little feared
That fate decreed and known to all:
Before the mightiest he should fall, before the mightiest wolf alone
That ever was whelped in cave or stone."
Lays of Beleriand
This refers to Huan, the Hound of Valinor, who is fated not to die until he should meet the mightiest of all wolves. The wolf in question turns out to be Carcharoth, who has bitten off the hand of Beren.
This to my mind says two things:
1: The mightiest wolf, that paragon of wolfhood to whom all wolves should aspire is the one who eats directly from the hand of Morgoth himself. It doesn't say the mightiest evil wolf, the mightiest warg, but the mightiest wolf.
2: There is an obvious parallel with the Norse tale of the Fenris wolf biting off the hand of Tyr. Tolkien is drawing heavily on the norse concept of wolfdom throughout the books, even naming the offshoot race, the wargs, after the norse word for wolf (vargr in old norse).
Set against this wolf on the side of light, is a wolfhound. A dog, albeit a big dog!
"At last, in the year when Earendil was seven years old, Morgoth was ready, and he loosed upon Gondolin his Balrogs, and his Orcs, and his wolves; and with them came dragons of the brood of Glaurung"
“Escaping goblins to be caught by wolves!” he said, and it became a proverb, though we now say ‘out of the frying-pan into the fire’ in the same sort of uncomfortable situations.”
Tolkien's wolves are those of the norse myth. They are the wolves of Grimm's tales who will gobble up little red riding hood. Peter's wolf who swallows ducks alive in one gulp. Robinson Crusoe's wolves, eating men alive
I've got nothing against wolves, but this is Middle Earth, not the Belgariad, not Call of the Wild, not Dances with Wolves. It's Middle Earth, where the wolves are evil.
I'll grant that simply because "neutral" or "common" wolves are not mentioned in the books doesn't in itself prove their non-existence. It is of course impossible to prove a negative. To accept that they can exist, however, I think we need to see something which gives the possibility. Something in Tolkien's background, the source material he draws upon and so forth.
As far as I can see, all the available source material leads to only one conclusion, middle earth wolves are by nature, evil. Evil from the earliest of times, allying with Morgoth and marching with him to Gondolin. There is simply nothing which points any other way, I cannot accept an argument which is based on nothing more than an unsupported opionion of: Well, there surely must be!