I've been asked this countless times. Consider a complete custom stock build, where a carefully built pattern becomes a "try stock". The customer can be fitted to it, adjustments made until it's "just so" - before the lovely walnut blank is even worked on. I believe that is a significant advantage and it's why I have so many patterns. Machined stock customers can probably find something in my inventory that suits them because of it.
Machining a stock has other advantages. I can chase the colour, figure and/or grain flow to best potential in each blank - something which cannot be done anything like as easily or well when working from-the-block.
People often choose cranky, figured blanks cut from walnut grown within areas of great tension or compression in a tree. Blank cutting techniques, seasoning and storage influence tension and stability in a blank. Early on, using blanks from various suppliers, I noticed that the stability from some was variable. This left me with some real headaches at times. Cutting my own blanks solved the greater part of the issues.
With wildly figured walnut, sometimes even the most carefully seasoned blanks are a little cranky and will settle a bit during stocking a gun or rifle. As material is removed the equilibrium of tension in the wood fibres is changed. When machining a stock, initially I always machine the stock oversize, with extra wood on both the outside shape and in the inlet. The stock is then carefully stored and allowed some time to settle if it is apt to. No second guessing here - the stock is monitored by recording reference dimensions off a granite surface table with a height gauge. By comparing measurements before and after resting, I quantify any movement and can take appropriate action rather than leaving this aspect to chance. Once the stock has passed this phase, it is re-mounted and carefully machined to final dimensions.
When making a stock from the block, a fine job can be done of letting the metal into the walnut - but the bulk of figured wood is still in the block. Shaping is done last, and if the blank is apt to move, the inlet is compromised. I love stocking from-the-block and get a real kick out of it - but I believe machining them the way I do is often a better way to make a custom stock, though it's more involved. In my own custom work, the art and skill involved in shaping a fine custom stock is still done by hand - but it's done on the pattern with any fine work also done on the finished job.
While most of the above relates to custom level work, the benefits flow on to machined stock customers. Other than a wide range of patterns, I can accommodate changes to Trigger-to-Grip, Drop and Cast as well as Length-of- Pull as part of a machined stock order. All that is required are those measurements.