There's another benefit of doing your own maintenance: You become more aware of the condition of your bike, and can switch to a proactive maintenance approach, rather than a reactive one.
Consider some items like cables and brake pads, possibly even chains and cogs. When someone rolls a neglected bike into a shop, it may have been allowed to deteriorate to the point of being inoperable. The shop needs to get it in and out, all at once. Also, shops tend not to allow themselves to make a temporary fix just to get the bike back on the road.
When you do your own maintenance, you become attuned to things like the condition of your brake pads and chain, because checking them is quick and virtually effortless. There's virtually no such thing as being caught by surprise, by a worn-out brake pad or cable, or having the bike become suddenly inoperable. Instead, we notice that a pad is near the end of its life, and order a new pair, but keep riding the bike. And if a part needs a tool that I don't have, I order the tool along with the part. Then I forget about it until the part arrives. I have never been caught by surprise, by a bad cable.
Personally, I don't think I need to keep anything on hand for my bikes, except for maybe a spare tube, and a bottle of the oil-that-shall-not-be-named.
And of course, there's still always that spare bike, just in case.
