I do use a custom XRay texture pack in my SSP, when I want to know if there are dungeons in the mystcraft age I created, for example.
As I said, as long as it is only used in SSP, no problem about that, saying that "you have X, Y or Z, why bother" is not helping. What is wrong with wanting a specific tool when you are the only one that will be influenced by it?
When you connect to a server, the server knows what mods you are using anyway, so you can be kicked or banned for using it, if the admins of that server actually do their job.
In SMP, you are going to influence others, as you would gain unfair advantage, so that is indeed not a good thing.
If all fails, you could install a mapper that support the search of specific blocks (don't remember the name, but it had the letter G at the end).