It is also relatively easy to get into a raid if you have an extra person who is willing to help you out. It really makes the restriction of converting your dungeon type (from 5 man to a raid) rather silly IMO.
I did state here that I know it's relatively easy to circumvent the restriction, and I have been doing so these past few weeks. My post mainly concerned the reasoning behind the restriction.
If the reason for it's continued existence is because it's always been that way, well that's fine. However, as we've seen with old-world flying and transmogrification, things do change and have changed in huge ways, even when Blizzard has previously stated that it will "never happen."
I suppose their main concern would not be old instance farming, but current raid tier, which I can understand. However, we saw after the release of Firelands that people are able to get into a solo raid anyway, so that argument is not valid.
I suspect it is a programming thing that affects multiple levels of your experience in a raid.
"Happens to a raid" or "drops for a raid" are probably tags that exist for abilities, triggers and loot that occur in raids.
And, absent these tags, the bosses would not use core abilities, would not correctly trigger events, or would not have loot.
I don't think this is the case, excepting perhaps for the experience of the person who joins/leaves, but not so for the people already inside the raid and killing a boss. I know this from my many years raiding, in guild runs and pugs, where various things happen concerning people coming and going. I've never noticed fights being affected in the manner you describe.