In addition to the advantages already mentioned, gaff tape has a fabric base and tears nicely in both directions, both off the roll and lenthwize so you can split a long piece down the middle if needed. The black variety is a flat non-reflective black that nearly dissappears if that's what you want. You can remove and re-stick it. I've blacked out a stand with the wider stuff. It's just so much nicer in every way than duct.
Like Moke mentioned the 2" blue tape works fine for cables, it's low tack painter tape so it doesn't leave goo and is easier to tape long sections and is usually less $$ which is good for that app since taping down cables uses up alot of tape. It's more available too but not nearly as strong.