The purpose of HST is growth. This is achieved by constantly applying an "effective" weight. The "effectiveness" of a weight is always relative to our current level of conditioning. That is why we can still grow by using weights the muscle has already seen before (after SDing).
We also want to be growing as much time as possible before having to SD. Very large increments will make the RBE "progress" faster as well (i.e. you will adapt faster). Sure, this can be effective (assuming there is no detrimental fiber necrosis, as mentioned by JV), but why not use a lighter weight which will achieve the same result? This way we still get the growth AND we delay somewhat the "progression" of the RBE.
When you can use a weight that allows you to do 10 reps and grow, why use a heavier weight that allows 5 reps only? The latter option will only make your cycle shorter (less time growing). That heavier weight will only be used when the lighter weights are not effective any more.
So, supposing that you are sufficiently deconditioned (so that the weights used during the 10's will be effective), you simply incorporate the 10's in your cycle to make it last longer and consequently spend more time growing.
Hope this helps a bit,
Dimitris
