This is one of the things I like about this game! If you are ever having trouble fielding a raid because you can't get enough healers or a tank to log in, you can get people to swap roles. So long as they are competent players, there shouldn't be any reason they can't fulfill the role. Perhaps not as well as a person that does it as their primary role, but if the skills are there, then making raids should be much easier.
It is weird how times have changed since my days in EQ. I can remember having to get 72 people online and ready to go... and doing it 4 nights a week without fail and this was before instances where we had to fight other guilds for content on 7 day timers. Nothing was guaranteed like it is nowadays. Now, in most of the other games I play we have a hard time getting 16 people to log on at the same time, much less the 16 we need, because often times we are short a healer or two, or short a tank (never DPS though, go figure). I really do miss large scale raids... this 8 - 16 man crap is dull.