This whole thing is probably moot given the reported promise to Jan Vesely, but here it goes...
1. It drives down JaVale McGee's asking price
If management has decided that he's truly the center of the future and he's not worth giving up for Derrick Williams, the Wizards owe it to themselves to do what they can to keep JaVale's next contract in check. Having Valanciunas around will give the Wizards some leverage in negotiations and gives them a back up option if McGee finds an offer the Wizards aren't willing to match.
2. Stashing Valanciunas makes the Wizards worse in the short run
This probably sounds like a negative, but consider that most teams in a rebuild have three years to put together their core through the draft. The Blazers and Thunder both had three year windows where they stocked up on lottery picks before they became playoff contentders. The Wizards appear to be headed on that same trajectory. WIth that in mind, why not subconsciously tank a little bit by drafting someone who won't be on the roster next season? It would allow the Wizards to optimize their last chance to get a major building block of the franchise via the draft before that phase of rebuilding ends.