How often have you wanted to SEE the results of a Pan/Crop in Preview but you've got a dissolve at the start/finish obscuring that first/last Pan/Crop Keyframe? And you've wanted to momentarily disengage that effect? I know I have!
Me, me me.
Does that count as 3 votes?
:-)
Seriously, what I do in that situation is move the event up to an empty track and preview my Pan/Crop move there.
Not the most elegant solution but it works for me.
Gets my vote as it's annoying when you have to wrangle it.
Maybe a better answer would be to have the ability to see the event in the source preview window kind of like AE's layer preview / workspace.