Usually I don't use pre-built themes or templates for building websites at LF Design, However, in recent times I have been sub-contracting for other web designers who use them to save time and money. Whilst an attractive option, it has some downsides and complications that I wish to avoid for my own clients.
You get what you pay for
Don't get me wrong, there are some fantastic pre-built themes out there for WordPress and other platforms that look good and work really effectively. However, I find they are very top heavy, over-complicated and often at times bug riddled. This is simply because they try to do too much. These commercial themes aren't tailored to a specific website so they must be very versatile to meet the needs of as many customers as possible. In consequence their code base becomes bloated and complicated leading to a high probability of undiscovered bugs. It also makes changes much more difficult and the user's browser must load more data increasing loading time.
Sometimes they just aren't any good
My last complaint about commercial themes is that you don't know if what you are getting is actually any good. One of the less perfect themes I've worked with was obviously not thoroughly tested across all browsers. Now that the website is live I've been fixing several annoying bugs that just shouldn't exist to begin with. One bug even disabled the contact form for all Firefox users!
I can understand why many web designers utilize commercial themes for their clients and along with the bad themes there are also some very good ones. However, I desire to provide my clients with the highest of quality which may not be achievable using someone else's code so that is why I avoid them.