Thanks to the introduction of account-wide achievements, GuildOx, along with any other site that is smart enough to extract this information from the API, can use the cross-account information to tell you exactly who that new player's alts are that's applying to your guild. So, if someone claims to have amazing gear, and anything else that isn't a linkable achievement on an alt, you can now check it out on GuildOx.
The functionality could allow a guild leader to see if the new person they're picking up is actually the worst trade chat troll on the server, for example. As GuildOx says, this can provide extra insight into applicants when recruiting new guild members. If you think you'd benefit from this, then you can check it out on GuildOx's new service by viewing one of the site creator's characters, and all their alts.
There is, of course, a down side.