I occasionally need to get a random number or the current time in my refactorings. In MySQL these are RAND() and CURRENT_TIMESTAMP, but in the other DBMSs it is slightly different. It would be convenient to be able to put something like LIQ-RANDOM-NUMBER and LIQ-CURRENT-TIMESTAMP into my changesets and let liquibase do the substitutions for me. (see http://www.statemaster.com/encyclopedia/Comparison-of-SQL-syntax for a nice comparison of a lot of the different syntax). I’m sure there are other simple commands that 99% of DBMSs have.
Right now I’m planning on using the modifySql tag, but that gets a little messy.