Guerillas are a 6/6/1 unit, Rifles are a 4/6/1 unit. Better attack but no better defense than Rifles which do also do not need a resource to build. That doesn't take defensive bombardment into account.
Aside from the fact that all of your old AA/MA units can upgrade to guerillas -if you are so inclined - the only reason is if you do not have the resources to make Cavalry (6/3/3) for attacks. By the time guerillas are in use, they will be up against Rifles or Infantry. The fact that they have the same attack rating as Cavalry is deceptive. With no multi-move or retreat options, you will lose a lot of them in attrition on an offensive campaign and the bombard on defense is, in my opinion, rarely decisive to a battle. I don't know if the AI takes defensive bombardment into account when attacking, but the AI only attacks if it thinks it has a chance of winning or in certain predictable situations (attacking a city, radar tower, etc). That means if the AI attacks your guerilla on its 20 year stroll towards the enemy's cities, it probably thinks it has a good chance of winning, which means you will lose units. Their slow movement makes them prime targets for skirmishing units. AI cavalry or even knights will take them apart without careful use of terrain. On the offense, attacking a fortified Infantry with any type of terrain bonus would be near suicide.
Generally the only use I have for guerillas is if I update a few old units for some boarder defense - a defense of a 6 is still descent for a while. You need to decide if it is worth the gold, though. Put them behind a wall on your second string defensive line or to quell rioters. I have, on the rare occasion, parked a few on a vital resource the AI wants - preferably on a hill or mountain. Build a fortress and let the AI
**Edit - one thing I didn't mention is that Rifles cost 80 shields, Guerillas 90 shields. Unless it produces unneeded waste or you truly intend to use them as multi-purpose units, might as well build Guerillas instead of Rifles (assuming you can't build infantry). An attack of a 6 might be useful and why not make units with defensive bombardment if you have a choice between two evils.**