Replacing an iPhone battery is easy, however sourcing an original battery which is warrantied against damaging your phone is somewhat impossible (unless you pay top dollar and let Apple change it).
With a lot of Samsung handsets you can purchase a genuine replacement battery which will be covered should it damage your phone.
If you're a heavy user it's nice to be able to bring your battery life back to 'as new' specs by purchasing a relatively cheap battery every 6-12 months. Samsung can assist here, and you won't need a screwdriver. This to me is an awesome 'feature' and no, it's not a dealbreaker. I have never had it down as my no.1 requirement from a phone but if Apple introduced the capability to iPhones then it would certainly be a plus point.
Oh and you wouldn't need a plastic casing! I'm sure that Apple could engineer a slide-in battery just like the SIM tray. They would rather you upgrade absolutely no later than every 24 months though...