depending on the magazine, i keep it. i have every issue of Make for example, re-reading back issues can come in handy. but if i had People, would I want last month's issue let alone last week's? not at all. my professional journals are very important to keep around for research even years later.
future-proofing is important, paper magazines are the most resistant to technology change because they don't need technology to read them. PDF has fortunately found itself turned into a "standard" format over the past few years, so magazines in PDF format are great. any other format has yet to be proven, even ePub can't say it's here to stay forever, it's still too new.
so the most important "feature" an on-line magazine/journal/book can offer me is that i'll be able to open it up in 10 years or 20 years, this includes the ability to take it to a new platform. i can still open text files that were written in the 70's written on a PDP-11. after that, the ability to search and index the file is important.