I could recommend plenty of prebuilt systems, and yes, they are fine, the lower entry level ones, for machines that are browsing or light office type applications only. The problem though is that any of them that come with hardware comparable to what you can build yourself, are all going to be more expensive, or if they are less expensive they are going to come with questionable hardware such as cheap, throw away power supplies that I wouldn't want running a light bulb, much less light duty gaming hardware such as their HD7750, or anything for that matter given the low quality nature of the power supplies, cooling configurations and the proprietary nature of most cheap prebuilt systems, such as what they are already dealing with, that don't allow you to replace worn out or faulty hardware later.
The convenience of just not ever having to do THAT again, makes doing it yourself more than worthwhile. It is absolutely unrelated to being a systems builder. Being a systems builder and long time repair tech simply affords you the opportunity to see how crappy most of those systems actually are, and how sad people get when you tell them you can't put anything better in there because they are proprietary.