As a client I have never hired any design firm without a breakout of project unless it was an extremely small task and therefore already clearly broken out. As Sagewing put it, seeing a breakout helps me decide what I want to use the vendor for and what I don't.
If copy writing is coming in at $75 an hour and my team has one we like at $55 an hour, guess what's going to go. That doesn't mean cutting away budget is the only goal, sometimes I find that we can trim one area and extend another.
I also find that with a breakout lets me double check the estimate against my own sense of the project. If 80% of the budget is spent on development and I'm doing a redesign we have a communication issue.
There's lots of reasons to breakout a bill but it should not be a burden to do so. On a larger project covering multiple areas, breakout categories will often include PM time, Code Development, Database, Front-end Design, Design Coding, Testing/ QA. If your project is smaller in scope (i.e. just doing design and build out) you may want to break down a bit further but you shouldn't be worried about trying to show every hourly task -- that would be an unreasonable request.