I can't really see any advantage at all. I think income trusts need to convert back to corporate status as soon as possible. Essentially, a large number of income trusts rely on the ability to raise capital. If they can't raise capital or debt, then they have to reduce their distributions, since income trusts are primarily valued on their cash distributions, even if they're holding their share price.
Income trusts are going to have to start operating like companies and stop distributing all their cashflow, and retain their cashflow to fund capital expenditures and support a low level of distributions. They're effectively going to have to start acting like corporations. Some of the better-run income trusts have actually been operating pretty much like corporations since they converted. They need to convert back as soon as possible. I see the four-year period primarily as the time period to allow them to do that, not really to continue to operate as income trusts.