Don't agree with this, unless there is hourly billing. As if you just want to try out some thing, say a large instance instead of medium, to just see how it changes something, won't be possible here.
edit: would love to know why the down vote?
In fact we ourselves, have not been able to go for a reserved instance yet. Although have been thinking about it for an year or so. As there is always a likeliness of going to a bigger configuration in the near future (small->medium or medium->large etc.). So you are not sure if reserving is a good idea.
So you settle for the gains of the on-demand e.g. having more instances in the day
Not a big deal if you are from the US but if you are from Australia for example then you wouldn't be able to take advantage of a Sydney availability zone if it become available (as it is rumored).