I know that text commands (as defined by \DeclareTextCommand) have
been important because of the various font encodings that arose over
LaTeX’s history, but it seems that the situation is quite different
for the XeTeX and LuaTeX engines.

Given that both XeLaTeX and LuaLaTeX accept UTF-8 input and use only
the TU font encoding, is it acceptable to forgo \DeclareTextCommand
for most—if not all—cases?

For example, ® can simply be included directly in the document source,
so \textcopyright is not much more than an ASCII alias.

As for less common symbols, one could simply wrap the text in a
document command.

Excuse my naîveté, as there are probably important advantages to the
text command approach that I’ve completely overlooked.