We need to step outside the trials world to find the answer to this, as in here the arguments are skewed by the unusual level of mechanical ability.
This is just part of supporting local sustainability, and promoting local cycle advocacy. Eventually, who is going to be left to fit that bit that you have bought cheaply online ... A decent lbs will support and co-ordinate riders on local cycling issues too, stuff like access, bike lanes, hosting rides, stuff like that.
More pragmatically, whilst I, and I dare say you and a fair number of other people who ride, can fix their bikes as well as, if not better than their lbs, and therefore don't need to worry about if there is a lbs or not - there are what: ten, twenty, a hundred times as many people who can't.
I was out riding the other day and a lad had come down heavy on his rim, and on top of all the other dings that rim had, this last one had pushed it too far to be ridable.
Now, we were only maybe a couple of miles out, so no big deal, but this kid had no idea of how to straighten a wheel and really wasn't looking forward to dragging his bike home.
So what does he do ? - sure, he can learn, or get his mates to help him (if they can) - but not there and then, and big mail order stores aren't going to help him out whilst he does learn ... his lbs would pull out the worst of the damage and only charge him a few quid to do it ... and yeah, I fixed it for him, but showed him what I was doing, so he could start to learn about it.
Lots of people aren't going to have the tools either - how many people have a headset press - ok you don't need one every day, but when you need one, and the block of wood trick just isn't worth it - you need one.
I'm not saying that I never do it btw, just something to think about ...
On the other hand if your lbs is rubbish, then they don't deserve the custom.
Start your own riders collective, and help each other out.
That's why ...
:D