The incentive for purchasing convertible bonds lies in the possibility of earning very high returns if the underlying common stock increases in price, making the conversion feature valuable. Because of this possibility, the investor in convertible bonds accepts a lower interest rate than that normally available on non-convertible bonds.
In a sense, the investor in convertibles is investing, not only the initial purchase price, but also the interest he foregoes by not buying a non-convertible security. The longer he has to wait to call, the greater the call premium or the increase in bond price must be to offset the interest foregone in the interim.
According to a recent theoretical paper by Ingersoll, a corporation should call in its convertible securities as soon as their conversion value — the value of the common stock that would be received in the conversion exchange — rises to the effective call price. This strategy, if followed, will severely limit the upside potential of convertible bonds. When subjected to the Ingersoll strategy, convertible bonds will offer a higher return than non-convertibles only if conversion value reaches call price very quickly.