Nothing wrong with cheaper prebuilt's if they've got what you want. Or locked i5's if they run your planned choice of games. Personally, I always custom build for a few reasons:-
1. If you install the OS from scratch then you've only got the cr*p you want on it and not at best, 20x extra pieces of "overly helpful" utilities / demos, at worst, glorified spyware. One of the worst offenders I've seen came with 2x anti-virus + 3x anti-Malware packages with real-time scanning enabled on at least 3 of them, no SSD and a 5,400rpm HDD. It ran like a pig due to constant I/O bottlenecks. One example of many of how you may end up needing to spend a day "tweaking" a pre-built anyway.
2. I don't know if it still applies (or varies from one brand to another), but the cheapest motherboard & PSU aren't always a good choice even if you don't overclock. Maybe you want to add a high-end video card down the line. Can the PSU cope at optimum 90% efficiency at 50% load, or is it "just enough"?
3. I'm a silent PC enthusiast quite happy to pay a small premium for silence optimized components (higher quality fans with better acoustic / vibration characteristics, over-sized heatsinks, low vibration / suspension mounted HDD's, gold / platinum rated PSU's and GPU's with zero rpm capability under light load, etc).
4. Better choice of optional extra's (eg, a certain size or brand of SSD or BD-RE drive which may not be offered as an upgrade). Or a certain design of case with maybe 4x USB ports at the front, etc?