Generally speaking, having defender is more of a downside , as the cost of losing the ability to attack often isn't worth it for having a creature with just high toughness
This basically allows you to turn everything on your board into mana, and Ashnods Altar is a key part of any kind of deck that likes killing off its own Creature for profit
If you pay the five mana and give an opponent a card, you get to make a 1/1 token copy if that creature was nonlegendary, enjoying that creatures undoubtedly killer EtB effect all over again
You can also attempt to limit dredges card draws, but this is a lot harder to do outside of Narset, Parter of Veils and the like
Thoughtseize If Modern has taught us anything, its that knowing your opponents hand is priceless
Or is this a case of the forbidden "partial payment" I found in the rules