My main concern about online apps? How reliable are they? Will it be a case of here today, gone tomorrow? We've seen websites that seemed useful come and go. And some become so changed they are hardly recognizable. Some start out free & change to for pay. I can also see some convenience in these sites. For someone who travels frequently for business or to give classes or presentations an app like Google Docs or something similar could be a way of not having to lug that laptop along. Store the the needed documents or presentations online and access them from any computer. Print your handouts onsite and not have to carry them with you. (I'd still have everything on a disc in my luggage.) The convenience I see is the same as for Delicious---having your material available from any computer. If using these products, I would still keep a copy of the material on my own hard drive or a removable disc while also keeping them online as another means of backup. I just find it hard to trust all this online stuff, know what I'm sayin'?
PS...I wrote this in Google Docs & published to my blog.