Changing fluid is changing fluid, now if people want to pull the cover and inspect, go ahead.
There's more to it than that. Changing the fluid is part of getting rid of the contaminants, which is the important thing.
I'm pretty sure that the same fluid could be reused (if there's no water contamination) once well filtered.
While sucking most of the fluid out is better than not doing anything, it still leaves the worst stuff at the bottom. At least I have never seen or heard of any device that doesn't.
Pulling the cover not only allows for an inspection, but to clean out all
the crap that has settled at the bottom.
Then refilling with new fluid (because it's so much quicker and easier than filtering the old oil) completes the job as it's intended to be done.
Of course, lacking a proper drain pan, sucking the fluid out and then letting what remains drip into a coffee mug when removing the cover works, too. But I don't mind spending $3 on a drain pan. They're reusable, just like the cover gasket.
As an aside, I recently heard about a guy who put on rubber gloves to pull the dipstick on his car and check the oil. It naturally made me crack up, but now I realize that he may very well use suction to drain the oil pan, too. If he gets that deep into vehicle maintenance, that is.