Unless you simply use a ton of machines, that alone will generate some extra electricity, but nothing noticeable, versus using a huge botnet to do it for you:

"The ZeroAccess botnet is one of the largest known botnets in existence today with a population upwards of 1.9 million computers, on any given day, as observed by Symantec in August 2013."

and

"These figures give some indications of the additional power requirements of bitcoin mining on a single computer infected by ZeroAccess. We can now extrapolate these figures out to 1.9 million bots and see what the total cost/impact is likely to be for the whole botnet.

If each KWh of electricity costs $0.162 then it would cost $0.29 to mine on a single bot for 24 hours. But multiply this figure by 1.9 million for the whole botnet and we are now looking at energy usage of 3,458,000 KWh (3,458 MWh, enough to power over 111,000 homes each day.) This amount of energy is considerably greater than the output of the largest power station in Moss Landing, California, which could produce 2,484 MW and would come with a corresponding electricity bill of $560,887 a day. Despite the costs, all this energy will create just $2165 worth of bitcoins a day! With these sorts of sums it would not be economic to undertake bitcoin mining with this setup if you had to pay for it yourself. But if the bitcoins are being mined at someone else’s expense, then that changes the picture completely and it becomes a highly attractive proposition."

source: http://www.symantec.com/connect/blogs/grappling-zeroaccess-b...

Once you make it attractive for someone to do this, they're going to do it, point blank. Add in no oversight or regulation and you have a recipe for criminals to come in and take over.