RE: Why do you have to wait until it completes its command?
I know a guy who wanted to make ABSOLUTELY sure his computer is powered down before he leaves.
REASON: His hard disk crashed because of a power surge when he forgot to power down his computer one night.
A very good reason to own a UPS backup battery system, maybe between $75 to $150 depending on power consumption. A good UPS will have two banks of power outlets, one is battery backed up, the other bank is not on batteries. However, both outlet banks will protect from power surges. Put the computer, monitor, external hard drives, router if nearby, on the battery bank and don't worry if you don't power down your computer. Cheap security to protect your data from crashes or surges (and brownouts). Another suggestion is to have a RAID 1 array. Get an RAID 1 enclosure for about $120 and stick a pair of drives in to mirror the data. If a drive goes out, your data is safe on the other one and you simply stick in a replacement hd to rebuild data onto the new drive.
No more "don't shut me down, I'M UPDATING!" crises.