A characteristic that has existed throughout most of UO's history (at least since 1998) is that the 2d client's development hasn't taken into account *lots of optimizations, small and large*, that the players have wanted.
UO Assist was originally considered just another 3rd party, potentially cheater program, and then it and a few others were accepted by OSI as being acceptable. UO Assist was/is one player-initiated effort to make up for some of the shortcomings in the 2d client.
Some of the unauthorized 3rd party cheat programs and mechanisms that exist today function along these lines, they clean up neglected client issues that the playerbase would like dealt with. Some of the stuff that is cleaned up is little, and some of it is big. An example of a little thing would be, there is a UO-Assist look-alike that queues actions for you (no more annoying "you must wait to perform another action" awkwardness in the client). An example of a big thing would be a graphics replacement that removes the graphics-intensive mage field graphics (para field, energy field, poison field) and replaces them with nonanimated ground tiles (a mod that PVPers prefer for several reasons).
The two above-listed things are cheats. However:
It's necessary to keep in mind that some player-initiated changes that were initially considered cheats have, over time, been absorbed into the client's functionality. For example, before UO Assist was approved and when it became necessary to disarm your weapon to cast a spell, auto-disarming that weapon upon spellcasting was considered a cheat. Today, the UO client does this for you automatically.
It would be smart for EA to take a close look at what the cheating PVPers are modding the 2d client to do and consider their own client changes in an effort to address the things these players appear to want.
Some of the ones I know offhand: The whole 2d client is sometimes accellerated in an effort to make player movement happen with less latency. Certain vegetation graphics are sometimes removed entirely in what appears to be an effort toward two things: making the game easier for the graphics card to render, and making it easier for a player to recognize and dodge around blocking objects in the world (the non-blocking vegetation objects are not even rendered). There's a graphics replacement for trees that causes generic tree stumps to be drawn instead, this presumably is easier for the CPU to render as well as is probably visually desireable from a targeting and navagation standpoint. And the well-known replacement of the generally intensive mage fields (para energy poison) with static tiles, I believe this is done to both help with latency caused by the animating tiles plus it supposedly allows easier targeting in the area of these fields.
You can probably tell that I've seen some of these things in action; this doesn't mean I've made these mods to my own client nor connected to EA servers with such things loaded. Having said that, I do wish EA would take a look at these kinds of mods (plus the functionality of that UO Assist look-alike) with an eye to understanding what's going on and perhaps building some of the evidently-desired optimizations directly into the 2d client.
2d client users do not want to go away, obviously, so maybe they should give them what they want instead of hoping to phase out their preferred client.
