Restricting any unit for a game like 40K doesn't work because, unlike something like MTG, you have a very limited amount of resources to make an army out of. Restricting cards in MTG doesn't mean asmuch because there are thousands of cards you can still construct a deck from, in 40K and FB you a limited to the 20 or so units presented in the codex. Restricting that one unit could hurt your codex quite a bit.
Units are "balanced" (a loose term in this case) based off the rules presented in the codex. To modify those rules reduces the effectiveness of said units to the point where they may not really be worth it to even play.
Think about it. A trio of Heldrakes are a badass team that will ruin another players day. Making a rule that the player can only take one makes his Heldrake a much bigger risk, as it will still be a huge target but can't produce the same results without it's team. The question then becomes is it worth the points or am I better off running an additional unit of X.
Actually, there is no "balance" - no matter how loosely you use the term.
Yes, the heldrake is badass. Yes, one will probably get shot down. But it's /so/ badass that, as MidnightSun said, the fast attack slot is "the heldrake slot". There are 5 fast attack choices in the chaos codex, all relatively good. But the heldrake is miles above the other 4. The other 4 don't get used in the highest levels of tournament play.
On top of that, the existence of the Heldrake with baleflamers basically maked some armies unsuitable for tournament play. Just ask Sethis.
Edit: in hindsight, I might be getting my threads mixed up. :p
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