1) They can be displayed on a bookcase. Seems simple, but I'm proud that I have read so many comics and enjoy the fact that when someone comes into my house you can see a huge array of material on the bookcase. Otherwise you drag some visitor down to your basement or attic (if you bother showing them at all) to see your 45 long boxes and show your collection. Want to share some great artwork, okay then let me pull out the box and then take the comic out of the plastic cover and show it to you. My method is here sit on the couch and look through this book. Better yet why don't you borrow it and read it.
2) The actual comic gets too valuable and you are afraid to touch it. I had some high dollar comics and it got to the point that I was afraid to actually take it out of the bag. Once you have a book that valuable you are no longer really going to want to take it out and read it, so how else can you enjoy the materials other then in a hard cover (or trade). If you want to hold onto the book and keep it as an investment fine (but I would say to sell it sooner rather then later). Also if you hold onto it make sure you have it listed as a separate item on insurance with an estimate of value from a comic dealer on record.
3) Ease in reading. Now this applies to Archives and Masterwork size books only. Still this is a great size to sit down where ever you want to read these stories again. The Absolute and Omnibus editions are nice, but tend to cut off your circulation in your legs as you are reading them.
4) The wealth of material that is now available. The Golden Age is represented by DC Archives with a vast array of material and Marvel has recently begun publishing their golden age material. The Silver Age material is almost blanketed by Marvel Masterworks and DC has a large array of material from that era also. Dark Horse has done Gold Key material for Magnus and Solar, as well as Tarzan material (published by DC) by Joe Kubert. Gemstone publishing is just starting to do the EC comics material starting this October in an archives format. Marvel has just recently started publishing their material from the fifties. All of it is really great stuff and I encourage you to at least sample some of these books.
If you buy comics and collect them to read first and foremost, then I believe hard covers and trades are the best way to go. Besides you can loan books out easier then a possible valuable run of comics and proudly display your comics to let people know that you are a fan. If your a football fan you might wear a team jersey, a paint collector has the painting on his wall, doll collectors have them in display cases - why should a comic collector hide his passion in a card board box.