When messing around with TikZ/PGF then I frequently find myself wanting to store a length for later use. By "length" here, I mean the word in its non-TeX meaning. As I'm doing stuff in TikZ/PGF then I tend to use
\pgfmath for the later processing so it doesn't matter for usage whether I store that length as a TeX-length or as a macro (providing I remember which I've done!).
So which is the better option?
The considerations that occur to me are: number of registers of each type, speed of look-up, speed of parsing, robustness of code (I feel that for a Cargo Cult Programmer such as myself then
\let\savedlength=\oldlength is safer than
\oldlength might not itself be a TeX-length). But I'm sure that there are others that I haven't thought of (being a humble CCP!).