Or divine star in the case of a fully stacked raid

Or divine star in the case of a fully stacked raid.... Sarcasm and condescension aside you'll still be perfectly capable of using any of these talents in a PVE situation whether they are instant or have a 1.5s cast-time. Since PVE encounters rarely feature interrupts the only change by pushing those abilities to a 1.5s cast-time is a minor loss of mobility and the timing, particularly for cascade/halo. All else being equal the HPM, HPS and healing output on those abilities remains the same.

You might wow or be able to say the 1.5s cast-time could lead to someone watching the encounter from the floor, whereas if they were instant this possibly wouldn't be the case. However, cases where an instant keeps someone alive but a 1.5s cast doesn't aren't incredibly common. In addition, these are AoE healing abilities we're talking about. You're probably timing the cast of those abilities to land just as the damage hits anyway, much like you do now.

The only current encounter where I could see it being a huge change is Thok. Of course that is a horrible concept they should never revisit for future encounters anyway.