Honestly...WHM only *need* 2 cross-class skills in 8-mans IMO (not to say only get these, just that you really should have them as your first and second cross class slots)

Swiftcast - Obvious

Eye for an Eye - Yes, it is excellent mitigation, especially on groups of enemies. The chances of it not procing throughout its duration is extremely low (just remember, it only triggers off physical damage, not magic...so it is useless on T2 and Ultima, for example)


For the other three slots...

Surecast - Its good to have, but not completely necessary. There are a few places where it does shine, however. T5, Ifrit EX (Howl can interrupt) and Garuda EX (and HM; I'm pretty sure Friction can interrupt, and know the main AoE does) are the three main fights I can think of where its good to have (whether you use it or not...different story). Of course, its just good to have should you take aggro and need to get that saving cure off. And, well, PvP...
Biggest problem for me with this is actually remembering to use it...

Virus - If you have a SCH or SMN in party...forget you have it. If there aren't any, its good to have for those times its needed. But in 8-mans you should have a SCH or SMN, if not both...

Bliz2 - Solo-play really...unless I was screwing around, I've never used this in an instance post-45 (Holy is so much better)