Slim is correct about the dirt.

Just don't fall into the idea that an Aluminum frame won't corrode. Unlike a steel frame, you can prevent the corrosion just by washing it clean. what I'm trying to communicate is that if you have corrosion happening to Aluminium components, it can go much much faster than it will with steel. I suggest the doubters go and learn something about electrolytic corrosion and the galvanic series before commenting further.

But even a steel frame won't instantly fall apart beneath you. My 'winter' bike is my touring bike, a very thin-walled 653 frame. It's over 20 years old. Bikes get nasty corrosion conditions here in the UK - year-round rainfall and a lot of rocksalt on the road in winter.

The LBS stated that you didn't need to worry about Aluminium, but you did need to worry about steel. They are right and wrong - both can corrode. You neglect Aluminium, let dirt and muck build up, you can get crevice corrosion in welds, and pits creating stress points. The Aluminium is then likely to fail catastrophically. Cracked steel will give way gradually.

I wish Titanium was a certain solution, but there does seem to be problems with quality control. I know a chap whose had 3 frames crack on the welds in as many years.