I guess it depends upon your mindset going in.
Functionally, yes you can used banked and borrowed points in a single reservation. If you buy 100 points, you will have 300 to use every 3 years.
But what if your desired reservation only costs 270 points? What would you do with the other 30? If the answer is "just let them expire", then I suppose you'll be fine. You will still pay dues on those 30 points and you paid money to acquire them in the first place.
Same with needing more points than you own. If you have 300 available (from 3 separate years) and need 330, you cannot borrow the extra 30 points. Most likely responses would be to either cut your trip down by a day, try to rent the points from another member or buy some "one time use" points from
DVC (which can only be used at 7 months.)
Personally I've never lost a single point in 9 years but if you're comfortable forfeiting unusable points every 3 years and/or adjusting your stay to fit what you have available, that's your prerogative.