There is great relevance in the OT, not merely as prophetic prologue to the NT, but also as proof that God's plan of Salvation was "in the works" from the beginning as well as the evidence and proof of God's faithfulness and provision to His people even when they were unfaithful to Him.
The same people who see no profit in studying the OT also hate to study history, because it doesn't seem "relevant". But without understanding history, we make the same old mistakes over and over.
If you study the ancient heresies you will see that they never went away, they just changes their names and the way they expressed themselves, but are as alive and infectious today as they were in history. The question of what constitutes heresy and why is a whole different can of worms. ;-)
Tim