If a buyer has a bid on the item and it is the only bid, then leaves a high proxy as security, this is not a good idea. The reason being that a seller can tell there is a proxy bid higher than the existing, as it will show as two bids, they wont know what it is, but they know if they are unscrupulous that they could throw in a schill bid and it will push the second proxy up. Also announces to other bidders there is a proxy.

This hold only when the bidder is the only current bidder.

Better to have one only bid. Unless it is deliberatey placed to eliminate a BIN option.