I can't tell any difference in resist rates. I run ACT and have a slash and hack job of a spell list (to use T9 masters and T8 masters, no appren's), and my resist rates on all spells are the same (most of the time, 0% resists).In my opinion it's always better to use the latest version of a spell simply because the mobs you'll be fighting (typically) will resist it less than the lower level spell...even if the stats on the lower level spell are better. At least, this is how the game worked last time I checked.
This is a fairly significant re-write of core logic that I'm not going to do.It's not the expansion, this has been the way useability works since the change in spell names. And to me it just makes more sense to know you can /usability "attack" and it will use the best possible version, rather than /usability "attack IV" at a particular level, and it would also greatly simplify the class spell lists.
Is it a big deal? Not at all. Is it something that could be used better? Very much so.