Working for an airline, I've always just booked my own flights travelling with the company I work for and the odd hotel here and there. However, I've recently been booking a holiday to Europe involving other carriers, several hotels, transfers, tours etc. I've found it really is a lot of work doing it all yourself online - especially when you don't know much about a particular area. It's taken more time that I realised and I can definitely see the advantage of paying a little extra and doing the whole lot through an experienced agent who maybe has the knowledge and expertise in the areas you are wanting to visit. Also, I'm guessing sometimes travel agents would have access to good deals that can't always be found online.
I suppose some people like doing all the research online themselves and they find it interesting & the planning is part of the fun, however due to working full-time, I've found the whole process has been taking longer than I expected. In saying that, I'll do it all over again and probably organise my next trip myself anyway.