Using a "unconnected" Battery to Jump-start a Car?

Since it is a nightmare to remove and install a new car battery,I have purchased the replacement,but as it is a busy weekend, I am using it as a jumper to jump-start my current battery when I can't start my car. So far it is working fine. Albeit a pain in the butt! I am also leaving them connected for a minute or two after so that I can recharge the new battery.
What are some mechanics opinions on this? Is this horrible to be doing? Is it OK,but I should obviously get down to it and replace the battery tomorrow? Or anything else you may have that can help me?
Also when I am jumping the old battery, I am not putting the new one on the cement or asphalt as I've heard (maybe a myth) that putting it on those two surfaces drains the battery.

Suggestion:

No problem, although leaving it connected for a couple of minutes is not likely to keep the charge up – five minutes would be more like it.

Bill Darden's famous battery FAQ says "In the early 1900s, when battery cases were made of porous materials such as tar-lined wood boxes, storing batteries on concrete floor would accelerate their natural self-discharge due to external leakage. Modern battery cases are made of polypropylene or hard rubber. These cases are sealed better, so external leakage-causing discharge is no longer a problem, provided the top of the battery is clean and free from wet or dried electrolyte and the same temperature as the floor."

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