Running obsolete hardware decreases the profitability of running newer, efficient hardware, preventing that efficient hashpower from going on the network.
It's a free market equilibrium thing where, long-term, the network isn't any more secure and you're losing money over a grid tie-in or from running efficient ASICs.
You might think "oh, well my hashpower isn't going to significantly impact others' decisions to purchase new mining hardware" - and to an extent, you're right. The impact's as insignificant as the security provided to the network.
/enthusiasm-quashing dick
A profitable alternative would be hosting others' efficient miners, who would be quite pleased to have access to cheap electricity (I'd guess you also have battery backup and the potential to use grid electricity if you had to, providing a relatively high level of redundancy, decreasing downtime).
1.5kW Grid tie solar (about 12kWhr a day in spring) + 300W wind turbine (planned) + 3 * 800W UPSs for storage and overnight running