The crank, flywheel, clutch are made to work together in a balanced way, the further something is away from the centre, the more effect a descrepancy is. For example, if the centre of the flywheel is out of balance e.g. you drill an extra hole, the effect is not as great as on the outer edge, the faster the wheel spins, an exponental effect will occur with regard to vibration. This causes extra strain on the main bearings on the crank until they wear and you throw a conrod.
Would you lighten the flywheel without getting it balanced? You can't just drill holes in places (especially the outer edge of a moving part) and expect everything to be fine. Unless of course it was designed to work with a different flywheel...
My friend ruined his brand new race engine fitting just the 6 bolts and decided after he rebuilt it (again - at great expense) to machine his flywheel and have it balanced with the clutch.
Users that read this can, as always do what they want, however to do it properly, the cheapest and easiest way is to use a 2.5 v6 clutch cover.