If you look back at some of the original discussion threads about the feature, you'll see why. I don't remember all of the examples, but I can think of at least two:
[Name] in particular can be an indicator that the lookup went wrong, but only if it doesn't get cleared (in which case you may never notice). As an example, say you import a file that is named "Bourne I", referring to The Bourne Identity and using a roman numeral I to indicate that. The lookup on TMDB for "Bourne I" actually spits out The Bourne Legacy (which is the 4th movie in the series).
If [Name] got auto-set on Import, you'd have no chance to see and notice the discrepancy unless you looked at the Filename. And, if you just trusted the lookup to be right (as I often do) you might have already applied RMCF, so you'd be even more lost once you tried to watch it.
Another example is explicit naming. For example, I have my Harry Potter films carefully labeled like this: Harry Potter 3 - The Prisoner of Azkaban. That way they auto-sort in my Movies view in the right order. When I ripped the BluRays, I named the files appropriately. Lookup works on these kinds of names, but the movie title is actually "Harry Potter and the Prisoner of Azkaban" (with no indicator of order). So, if it changed [Name] on auto-import, I'd have to go back and manually fix all of these every time they were imported. As it is, it "just works".
Leaving the [Name] field auto-filled (by tags, or by Carnac's usual parsing) is just plain safer. Fixing it, if you do want the name provided by TMDB or TVDB, is a simple matter of running the tool again manually.