lucy24 - 11:59 pm on Feb 21, 2013 (gmt 0)
If you are hit by a truck, how hard will it be for the client to replace you?
The client may not think of this-- though they should-- but as a professional you should definitely take it into account. Same as with any contract: What happens if for reasons beyond your control you are unable to do the work?
"I use a terrific programming language that I developed myself and nobody else in the world knows how to use"
"I use an out-of-the-box CMS that has its shortcomings but any idiot off the street* can pick up where I left off"
... and then you work out where on the continuum you fit.
* Possibly including the client himself, though you won't put it in those words ;)