This is already the case and will not be changed.
No, *not* everywhere ! I already coded the viewer so that legacy names are *always* presented in the Profile floater, whatever the setting, and things such as money transfers don't have anything to deal with (or benefit from) display names (which can even be dangerous: imagine someone using your Legacy name as a display name...). This is a step I will *not* take in the Cool VL Viewer: money transfers, inventory offers, inspect and object properties floaters, about land floater, mute list, etc will all retain the legacy name as the only identifier, since it's the thing that will never change for any giiven avatar in SL (when display names can change every week).
The only usefulness of display names is to be able to change it for RP purpose (so that your avatar can have a name matching your exact wish for a given RP), but even there, it fails short for people like me, who can RP in half a dozen of different RPs (with different storyline and avatar appearance) with the same avatar in the same day (and since you can change the display name only once a week, it's pretty useless !).
<ranting mode>
In fact, the reason why LL implemented display names is just because they think that they will get more users registering to SL if the registration form only got one box to fill in with a username of their choice... LL forgot two things: since there is only one field for the username (instead of two fields with legacy names, one for the first name and another for the last name), the potential new residents that were giving up before with the old registration process will still give up on the new registration form after entering a few variations of a username (try registering "John", and you'll see if it's not taken already...). They also forgot that if a person is not willing to spend some time to fill up the registration form, they will not bother with SL either and will give up after the first login never to come back again (the learning curve for the viewer and world features in general is much more demanding than the effort to fill up a simple form with two fields for the avatar name...).
The very damaging result of this very stupid idea of some pretty ignorant and illogical person (or group of persons) in LL's staff, is that now you will have John1234 and John1324 on the grid, and good luck to distinguish them from their username !...
Another problem is that people won't be happy with IRC-like names in SL, and that's why LL made out the whole idea of display names, also arguing that people always wanted to be able to change their name (when the actual need was to be able to choose whatever lastname you want when registering a new avatar, like you can do on almost all OpenSim grids)...
</ranting mode>
In any case, IMs and chat are *not* the same thing, especially for role-players, but also in groups and for friends: display names can eventually be useful in chat, for RP purpose, and for the poor new John1234 Resident (so that they get a decent avatar name), but since display names can change at the whim of the avatar owner, and since IMs are most often used for "serious" stuff, you (or at least *I*) need to be able to keep things clearer and stable for the identifier of the person you communicate with.
However, there might be cases where you might want to use only the display names in IMs (for example when you must switch to IMs to RP with John1234 Resident and want to see him as John Doe while RPing in the IM window). That is why I am currently planning on using the same logic as for profiles for IMs (and friends list), i.e. to show the legacy name in all cases, but also plan on adding a small check box so to switch the setting of a specific IM window to display name only. I'm still not decided about the friends list (eiher still showing the legacy name in tooltips, or allowing to switch the NAme column back and forth between legacy names and display names with a check box ?...).
Not sure I will implement this either: the reason for the choice of the "Display Name [Legacy Name]" format is that parenthesis are already used for "Display Name (user.name)" (reminder: display name can be whatever, legacy name is Henri Beauchamp, user name is henri.beauchamp) and also, in avatar names bubbles, for the Away/Busy indicators. Also, when you enable display names, it's only logical to show them first (especially with the new residents who will have little choice but to use display names), and only the legacy name between brackets as a mean of trusted identification (invariable identifier)...
Things are not yet settled, so I may still change my mind on this, but it's what I came up with so far...