Has nothing to do with being able to tune it. Has to do with a couple different issues with the factory style 3 circuit metering blocks. One is the inability to lower the idle feed restriction. Early 3 circuit Dominators had the idle feed tube pinched at the bottom, this was your idle feed restrictor. However it made tuning it impossible without pulling the tubes. Later Dominators use an open tube and made the restriction at the top, above fuel level. This is not as stable for fuel delivery as discussed in the link below.

Next is the obstruction the idle feed tube becomes when either higher HP or combination of higher HP and oxygenated fuels are used, you can reach a point where adding more main jet has no effect. Some companies throw a bunch of intermediate fuel to compensate, that is a bad way to do it. You get more liquid fuel, and it will usually negatively impact distribution. You can open up the main wells, but that means pulling the tubes out.

And last is the need to even have intermediate fuel. If you tune it correctly the need for intermediate fuel goes away for most applications. It's like having dirt hauled in to fill a hole you don't have. You end up with more fuel than you need and that can foul plugs.

I use two types of metering blocks, BLP 5124 2 circuit or 5120 3 circuit wide body blocks for gas. I will almost always plug the intermediate circuit on the wide body blocks, I use them sometimes because I can control the size of the kill bleed. For methanol I use a methanol version of the wide body block, and will use a little from the intermediate circuit only to supplement the mains if needed.

http://racingfuelsystems.myfunforum.org/viewtopic.php?f=25&t=18