I think it all comes down to 'how the classics are used' rather than 'should I' use them. I've heard tons of sets that use classics that are horrible and DJ sets that never use classics and are always great.
IMO a classic is best used in a set when:
- There's enough meat left on the original to clearly identify it as a classic (minor changes)
- It plays long enough to enjoy it, but short enough to not hear the whole damn song. (I hate hearing classics that only play for 10 seconds, but def. don't need more than 45-60)
- It's played in its pure form to some extent, and then the DJ bridges the 'gap' with a remix, breathing in new life and taking it to the next level. The purests get to hear the track in it's original form, and the remix ensures the newer gen. isn't left behind.
And as for Tiesto's adagio, there are tracks that are toooo distinctive and must be used sparingly or altered enough in the remix to be tolerable. Adagio has been played enough that we all need to take a break from it, or hear a reeeally good remix (of the remix).
My $.02
-pand