An example - in key C major, the E7 chord is a 'secondary dominant' chord. Now, if it goes directly to the chord it's dominant to in its own right - A, or Am, then it makes sense to call it secondary dominant. However, if it went straight to F, quite a common occurrence, it ought to lose its right to be called secondary dominant, as it's not actually posing as such. We call it secondary dominant because of its role at that point in the music, but it doesn't fulfil that role. So is there a better term to use, as it appears to be a misnomer, not really describing its function?
And, with the same theory, is a tritone substitute a secondary dominant?