I suppose it wouldn't detract from the value if the carbine is not particularly collectible and/or if it's so worn or rusted as to be unsightly. However, it may not add any value either. In fact you might not be able to recover the cost of parkerizing. I guess you have to decide if it's worth it to you personally. That's my opinion.
A lot depends on what you are reparking? Original carbine w/original parts? Import marked? If it is a rebuild, as most are, the current condition of metal might be more sought after then a reparked. If it is an orginal, as issued carbine then hell NO!! It really depends on what YOU want of the carbine for your enjoyment or concern for re-sale. If it is in poor condition then a repark, if done right by the right person, will help. If metal is in honest patina condition then reparking will hurt value. Shuffs may be the man you want to have do the work if so desired. You must be logged in to see this link.