Agree Karaya - in fact, since I often am 600 miles away from the shop I use, there has been a couple times when I've wanted/needed a new system, that it's better for me to pay the 60$ - which they wave for me and other long term customers - to have one of their wonderkids assemble it for me. This is good for a couple reasons - most important being RMA/DOA stuff. When 600 miles away, having a component not work out of the box is a giant giant PITA. Where as, if they are building it and testing it for 2 days as they do, they can instantly replace it without playing tag in the shipping department. So, I can understand if you're dealing mail order from a distance that having a "you build it" option isn't all that bad.
It's not that I can't or don't want to assemble/build, it's just simpler, sometimes, to have them do it for me, depending on where life has me living at the moment. Most places in Canada offer this service, I'm sure it's the same in the USA - I've used the same 12 store chain since they had a single 2 room office store in Calgary back in the 90s, Memory Express, and they will build it, test it, install all the latest new drivers/etc, however you want it, for 70$ or 50$ USD. NCIX and the other companies up here will as well.
There are many reasons to build, and not to build, it just depends on the individual and the situation. In my case a single defective part means about 2 business weeks of waiting, as getting the RMA, shipping it back to them, them being so busy it takes 2 days to receive it, then the turnaround, etc, it's been Friday of the next week the last time that happened. Why go through that again when they can assemble it all for me for free (50$USD for non long term customers) - and they know I'll check the work as I know all the builders there, and they have been 100% for 3 of the times I've had em build for me. Still, when I'm IN Calgary, I always prefer to do it myself, as I just prefer to as I always learn something while I use their shop doing it, and it's fun. I do think it's something every serious PC user should learn to do, even if by trial and error/Youtube/etc, but again, I understand completely why because of circumstances many do not.
edit - I would also say that the shop I use, as well as the competitors, up here have a price match guarantee - I can use PCPartPicker, build a system, and they match it immediately, even if they are building it for me. So if you find a good "local" shop like mine, which offers this, price isn't really an object because they will meet or beat any posted price.