No, not necessarily.I never thought about that good point... Adding the second drive will increase the noise, heat and power useage.
One WD Scorpio Blue 1 TB 12.5 mm needs 2.5 Watt (5 Volt/500 mA). The WD Scorpio Blue 1 TB 9.5 mm needs only 1.4 Watt. Newer Samsung SSDs (840 & 840 Pro) need also much less power, compared to older SSDs, like the Samsung 830. So if you replace your old HDD with a combination of a newer HDD & SSD, you should not see a reduced battery life.
----------
iAgree 100 %.Minimally, unlikely you would ever notice it
----------
iAgree 100 %.I don't believe a 5400rpm drive would use more power and/or be noisier. I could possibly see that in a 7200rpm drive but, I can't see it with the slower one.