I revisit this only because I see this topic pop up on the forum regularly. After today, my fears of drilling with my HSD spindle are officially laid to rest.
I took the opportunity to stop by the HSD booth at the IWF and asked in person "Will using my spindle for drilling damage the bearings?" The factory rep seemed puzzled by the question and I clarified about drilling shelf pin holes, predrills etc... I told him I'd heard that using the spindle for drilling would shorten the bearing life if not destroy them. He asked where I had heard such a thing. He assured me the spindle was designed to take pressure while drilling. Not deterred I pressed the issue.
After bringing in a second rep and then calling an engineer, lest there be any doubt as to the ability of the HSD spindle to withstand "drilling" pressure the 4HP spindle (my flavor) is used in a mortiser (yes, a MORTISER) which spends its entire life under "drilling" pressure. There is no danger of destroying your bearings just because you drill a bunch of shelf pin holes. Abusing it is a different story as with anything. High pressure drilling with inappropriate bits of course WILL damage bearings. Not all bits are designed to bore straight down and should never be used in that manner as they can start fires as well. It still sounded as though burning your shop down would be a greater danger than destroying your bearings.
While I'm sure there will be someone of a different opinion (which is fine) I for one am of the opinion that drilling pressure will not harm my spindle.
/RB