It was the first Valiant title that was a new property (after Magnus and Solar, and a month before X-O Manowar). I think the value has to do with a number of things.
• As mentioned, it has condition issues, probably stemming from the coupon insert.
• Coupons were removed from a number of copies to redeem the limited pink version of Harbinger 0.
• Hype at the time. In his editorial in issue 1, Jim Shooter compared the book to Avengers #1. (A bit of a stretch, but clearly Shooter thought more of the property.) I don't recall a similar comparison being made for other titles.
• First appearance of the 5 main characters. Other original Valiant creations were teased in other books. The X-O armor (but not Aric) popped up in Solar. Rai debuted in Magnus. Jack Boniface had a cameo in X-O before becoming Shadowman in his first issue.
• The story at the time was a huge departure from other mainstream superhero comics at the time. In my opinion, it doesn't quite hold us so well on a re-read now, since comic storytelling has evolved a lot since then, but it was unique for the time period.