I think bidder handles/usernames are unnecessary, and would cause problems.
1) Some people will use fake names, or they will use names that are the actual names of other people/companies. And it's not fair for other people/companies to have their name used like that.
2) Even if you just assign each account a number, then eventually some bidders will have an idea of who is who. And let's say you do a lot of buying, but aren't a "big fish". But if some "big fish" notices you are bidding on some domains, they may try to push you in auctions to try and force you to pay extra... to basically deplete your funds, so you have less to spend in other auctions (so that big fish will have less competition in the other auctions).
3) Also, big companies like HugeDomains, BuyDomains, etc. are sophisticated enough that if they notice certain people are good at finding gems, they could use software scripts to scour auctions and see what those people bid on... and then enter the auctions to outbid them.
People already say that big companies like that already do a lot of bidding using software (artificial intelligence to some extent) and bidder IDs will make it even easier for them, at the expense of "the little guys" who do the work to find the gems... and that's not fair.
If bidding isn't anonymous, then people can be targeted like that.