So there a a few different types of shop mechanics:
Those who think it's a job:
Horrible people, these. An entry level mechanic makes minimum wage, has absolutely no clue what they're doing, and is likely in it for the shop discount. Good deal for him, bad deal for you.
Those with an agenda:
These people love bikes, and think that everybody should get rid of their car and ride a steel bike with 700x32 tires and racing is stupid and anything newer than 6-speed is stupid. They often know their shit, but not with new equipment, and they despise you for making use of technology that's any newer than the printing press. If your mechanic wears a cycling cap indoors that looks like it's a bazillion years old, this is probably his category.
MTB suspension and braking systems require a fair amount of expertise, so anybody who is skilled at fixing a mountain bike should be able to handle anything that goes wrong on your road bike. The issue is that they don't care about your road bike, and they're likely too buzzed to get it done in any reasonable amount of time.
The ex-pro mechanic:
The guy that's been in the industry forever. He probably worked on Sean Kelly or Andy Hamsten's bike back in the day. They race masters, and simply love working on bikes. Somehow, they are at a place in life where it's ok to work at a bike shop full time. They keep updated on new equipment, and can fix anything with anything. These guys are awesome, but they're few and far between, and the shops that they work for are going to be pricey. They're also probably quite busy, so anything more complex than a tire change is going to take your bike out of commission for a week or more. You don't find these guys at performance, or your local trek/specialized concept store. Actually, if you think your mechanic is like this, you're probably wrong and you're probably getting overcharged for shoddy work.
Unless you have way more money than time (if you do, please accept my jealousy), you don't have any use for a bike mechanic.
With a modest number of tools, you can do 99% of what a shop can do, and you can do it better. What bike shops have in terms of experience, you can more than make up for with the fact that you actually care about your bike.
You do care about your bike, don't you?
Ok the tools you need:
A set of allen wrenches, 2, 3, 4, 5, 6, and 8mm Here's a cheap set.
Chain tool, Like this one
Cassette removal tool, and chain whip 5 bucks, 12 more
Cable Cutters (wire cutters will NOT work for this)nice ones will make your life a lot easier
Rags, You can figure this one out.
Your chain lube of choice , Everyone has one, this is mine.
Grease, you can get a 1lb tub from an auto parts store for less than $5
A phillips head screwdriver, you know what those look like
A set of needle-nosed pliers, those too.
A bottom bracket wrench, like this, but they also come with shimano cranks (bet the shop didn't tell you that!)
Ok that looks like a long list, but aside from building wheels, this will accomplish EVERYTHING you'll ever need to do to your bike. Check out the www.parktool.com website for tutorials on how to fix or adjust anything on your bike and you're good to go.
Knowing how to ride a bike is great, but understanding your machine makes you a true journeyman. It also gives you the power to help others should the need arise. So stop wasting your money and losing your bike for weeks at a time, and learn the tools of your craft.
Want to know how to do something specific? ASK! I can tell you how to do it, or I can point you to a website that does it better than I would.