In more ways than one! Self-generation is also a problem at the distribution level. Any source connected to the grid is a potential hazard to lineman.
Picture the typical storm scenario - wind blows through, knocks down a few trees, lightning strikes and blows the occasional insulator or fuse. The lineman comes out after the storm and sees that a tree had fallen, causing a fuse or circuit breaker a mile away to blow or trip, resulting in a power outage. His job is to get the tree out of the wires and get the wires back up, and then restore power.
But he won't start working just yet - first, he has to open or check open and red-tag all switches that could possibly energize the line he's working on. Pretty routine today, when all possible sources are mapped and are generally few in number. Perhaps two or three switches and he's made it safe to ground the conductors and start working. He will not work without a proper tagging procedure - it isn't safe to do so, and it's in violation with OSHA standards that have been in effect for decades (back when OSHA really meant something).
But wait - what if 2 or 3 or 10 or 50 customers have their own power source. Now, the lineman has a choice. He can go to each home and disconnect the meter, and tag, or he can hang working grounds, potentially damaging any generation or power source that has been inadvertently connected to the damaged part of the distribution system. I know what I'd do if I were a lineman wanting to restore power to my customers.
Incidentally, this problem has cropped up on occasion when people pull out the portable gasoline-powered generator after a storm and proceed to backfeed the distribution lines. Most people don't even think about installing a proper transfer switch to prevent feeding the incoming line, and it's evident that some don't even bother to open the main breaker before connecting their generator.