I agree that real books are to be preferred & I feel the eBook in its current form certainly isn't a substitute. But I can think of a number of advantages for products like DS developing this over time which could easily sit alongside either owning books - btw, something potentially quite profitable if you know what to look for in the 1st editions market & more than a passing interest of mine - or, as you say, lending them from libraries. For many serious readers there often seems reason enough to want to refer back again to favoured old books, rereading them, etc., so having these always at hand is pretty useful.
Yes, the Kindle's library seems to be growing fast & certainly looks "interesting", but (AFAIK) it hasn't reached the UK yet & for now at least I'd prefer any eBooks I might buy on something like the DS, which offers some quality gaming to boot. I won't buy any eBook reader right now, but it's conceivable I could change my mind within the next year or two.