From what I understand You have already working web UI for human tasks, so You should already have support for showing available actions like suspend/resume/finish etc... than You actualy dont need humantaskservice. You only need to put in your services 2 methods for starting and finishing human task, just implement humantaskhandler. Beside that get some human task state object to your model to save intermediate states between start and finish.
Adventages? You dont need to stick to presetted human task flow which You can't change. Every company and even every task has different stepps possible and not every body can be suttisfied with standard solution.
You could even use simpler and yet possibly more powerfill option: use subprocesses instead of standard user tasks, those subprocesses could consist of different possible human task flows so you even dont have to implement complicated flow login int your code. Here human task implementation would be simply start/finish support. Flow would be taken care by designed process which is better for business ppl who could change that human task flow or create new ones when needed whithout help from technical ppl or even server restart.