For those who are interested, it would seem that the point of the Umbrella and Darkside Chronicles is basically to cover all of the Resident Evil content prior to Resident Evil 4 and 5, and they decided to do this through the simple method of an arcade shooter. While the gameplay in itself won't be overly enthralling for those who love the puzzles, adventure and horror of the Resident Evil series, it will provide the background for the much acclaimed recent games without having to pick up a wealth of other games.
Indeed you'd have to go through all of RE Zero, RE 1, RE 2, RE 3 and RE Code Veronica, whereas all of this content is given in these two games with additional playtime on events leading upto RE4. Of course all of the games prior to RE 5 are available on the Gamecube, but if someone was looking for a cheap easy update on the background then these arcade shooters would certainly be the way to go.
They are arguably on the rails to too great a degree - you should at least able to look around - but for mindless fun which also provides you with the much wanted background they're a good buy. I'd really advise it as a two player game myself, because merely shooting at a screen on your own for hours upon hours can become extremely tedious.
Thus I'd only go with them if you love on the rails shooters, you're looking for a simple game to keep you and a friend entertained or you're enthralled with Resident Evil but need to know more without major investment!