Nope, I know exactly what their fees will be. It's more than worth it for peace of mind of not having to deal with my sister. I'm an accountant and have seen, far too often, the fallout from family arguments about money, especially on death, and especially when one or more beneficiaries simply won't listen to what the law/will says, or don't understand it, claiming the will means something completely different. Unless you've been involved in some of these disputes, it's hard to understand how much conflict and argument they can cause. Even things clearly written in black and white can be argued by some people who simply won't accept it because they've been told by "a bloke down the pub" or read something on facecloth or twatter, something completely different.
I've seen disputes with clients, some going to court, where an aggrieved relative has tried to claim all kinds of things, i.e. not of sound mind when making the will, mistakes in the drafting, claims of executors making mistakes, and of course procedural errors when making the will, i.e. not getting it witnessed or getting it witnessed by a beneficiary, or it not being dated, etc etc. I'd never either write a will nor be the executor, without engaging a solicitor to ensure it's all done properly and to keep potential aggrieved family members at arm's length!
Horses for courses, you have your opinions, I have mine. Of course, lots of people can write their own wills successfully and act as executors without problems, but I can assure you, lots of others don't have an easy ride, even though in theory, it should be so simple!