The question in this case is more appropriately, “why would you choose an expansive bit over another set of drill bits?” as that is where the expansive bit’s real strength lies. Read on!
Advantages
Always be prepared
Expansive bits can be set to bore holes of a huge variety of widths, as small as 16mm (5/8“) and as large as 150mm (6”). This means that you would have less need to purchase other wood boring bits, unless you needed one that was smaller than 16mm (5/8“).
This saves the DIYer from having to buy a set of wood boring bits, and therefore saves both money and space.
Disadvantages
Neatness
Due to the way the bit’s screw tip and spurs are constructed, there is likely to be tear out on exit. This means that, as the bit leaves the wood, it will tear through the wood fibres, creating unsightly splinters. However, this can be avoided by stopping drilling as soon as the guide screw breaks through the reverse side of the workpiece and completing the hole from the other side.
Even if the cutter is set to the right length on the measurement gauge as accurately as possible, it is not unheard of for expansive bits to cut slightly larger holes.
While this is unlikely to cause a major problem in most woodworking projects, joinery relies on accuracy and large amounts of work can be ruined through drilling inaccurate holes.
Not easy to control
Because of the asymmetric design of the expansive bit, with one long outrigger protruding from the centre with a much smaller primary cutter that is not always located opposite, it can be difficult to keep the bit moving. This can make for a jerky, undesirable experience thanks to the friction caused by dragging the outrigger spur and lip through the wood fibre, which can cause damage to the wood.
Need tools for adjuster screw
Even though you save money on drill bits by buying a set, some of that money will have to go on buying a screwdriver or socket wrench so that the lock screw can be activated.
Depending on whether or not you already have a screwdriver in your toolbox, this might influence your decision on whether or not to buy an expansive bit.