I've used a small piece of string (by random chance, I have a very thin nylon sheathed spectra line on my keychain at all times) to open a link--it's very easy, and is essentially doing what masterlink pliars do. In my mind a spare link is mostly to fix some major trauma to the chain and replace a damaged link. I carry a multitool with a chain tool for removing damaged links, and I pretty much always install links by connecting the loosely on the bottom run, running them up to the upper run, and pressing on the pedals with the wheel held stationary.