It's mostly due to the complaint of excessive tab reloading in Safari - which is a valid argument.
However, I am actually wondering if a RAM upgrade will really fix this. I have the sneaking suspicion that it is more of a problem of iOS and/or Safari, because you don't need hundreds and hundreds of Megabytes to store the contents of a few Safari tabs, and if it were so bad on the 5/5S due to the RAM, then my 4S (with 512MB) would have to be completely unusable with tabbed browsing - which it isn't. So while there is something to be improved in the web surfing experience on iOS devices, I somehow doubt that additional RAM will change anything, but that seems to be the basis of the whole RAM upgrade discussion.