|Don't ask to ask, just ask|
The web server handles all logging in terms of access and error logging for most CGI scripts.
If your using DBI the database logs all data with greater efficiency than a hand rolled solution.
Do you think in these instances logging is therefore unecessary?
Depends on whether what gets logged is what needs to be, I guess. :-) In general, I'd say no. These logs describe what happens at an interface, and misuse of an interface isn't necessarily the same as assumptions being violated. When it is, its signature can be quite unhelpful.
For example, maybe I'm updating a database through DBI, based on a key built from several bits of data. I pull these different bits out from the input record with regular expression captures, cat them all together (somehow) to build the key, and then do a $sth->execute or similar. Seems reasonable, right?
If one of my regex matches fails -- maybe the input data are corrupt, or one of the upstream programmers changed the record format slightly, or something -- the best I'm likely to do is get a "use of uninitialized value, foo.pl line 666" warning from perl. I'll build a nonsense key, then try updating on it -- and updating on a key that doesn't exist isn't an error, last I checked, just a null op. Now maybe that warning makes sense to me, but if I'm catting three different strings together, any (or all) of which might be the problem, it's going to take a lot of unnecessary effort to track down the problem.
If, on the other hand, I do something like:
then I know exactly what's wrong, and how it's wrong, just from looking at the logs.
To me, that's a big win.