I like using a topsider as it makes the oil changing process cleaner, easier, and faster. The only reason I can see to remove the drain plug is in the event it’s magnetic, to see if there are any metal shavings attached. And, of course, to softly weep when you find a large collection of filings…
Of course you probably won’t find this unless you don’t change yer oil often.
I would also wonder if anyone knows the rate at which engine contaminants settle out of oil? This would decide the % of contaminants that would likely be stuck to the insides of the engine, and bottom of the oil pan.
But then, it wouldn’t really matter whether the oil was suctioned out or let gravity remove it. As you’d have the same amount stuck to the inside of the engine in either event. As the oil level in the oil pan goes down, the same amount of contaminants will be removed, or not, in either event, except perhaps for that last tablespoon or 3 at the bottom of the pan. Still worried about getting the last nth of dirty oil out? Try putting another pint or 2 into the engine and then remove that in the same manner. Either way, what ever is left over will be diluted by 6-12 quarts of new oil.
The entire debate of using a Topsider or similar device as opposed to using the drain plug is about the same debate as using an abacus or a slide rule versus a calculator. In the end, the only thing lost by using a calculator, or a topsider are the time consuming and overly work intensive rituals required by the previous, antiquated process.