You're right that the use of rank 1 Life Tap is going to be impacted by the removal of spell ranks but this is a pretty minor concern, and only really one for healers - especially those who have an issue with locks who Tap for mana. And I don't care about them.

What we have at the moment is a situation where a minor inconvenience for a healer (and yes, I have played a healer) can be made much more minor by the use of a lower rank. The only time it is worth doing this is when you don't need any mana but want to proc the glyph. As has been mentioned, the glyph is liable to be reworked but there's no reason to assume it won't still be around to give a combat benefit upon tapping. So we have a situation where a minor effort is offset by (essentially) no effort on the part of the lock to downrank. Minor trumps none so it's worth doing.

A single full tap is almost never an issue. The bigger issue is with the activation of effects like that of IDS where several taps are required, but then other classes are in this boat and they don't even have a tap to offset.

I am far more interested in the removal of spell ranks from a leveling point of view, I think it's great, but that's not really anything for a warlock post.