Single tower not good?

Are single LLT/Defender that good, if you rush more than 4 glaives vs it, more than likely it will die.
Before I play the teammatch, should I concentrate towers on one point, or spread them out to defend more space and more economy?

That question is totally situational therefore impossible to answer but I say spamming defence towers is always a good thing as long as you are expanding and using them to defend expansion.

e.g: every time you take a metal extractor build an llt next to it

The problem with spreading your defence too thin is that a grouped enemy army will just waltz through it. If you have a clusterfuck of towers then it will do some real damage if an enemy army walks into it.

I personally like to spam towers everywhere though, so I say go for it! Using a mixture of llt and missile turret is nice too.

Interlocking light porc + solars can be suprisingly hard to over-run - I remember a game on red comet where I was trying to over-run Drone in the first 6-7 minutes (who was opposite me and had pushed past the middle onto my side with porc creep), his compo was a mixture of support com, slasher, wolverine, defender, LLT and awkwardly placed solars. I was hitting it with a battlecom & mixed forces of Ravager/Riot/Scorcher, and getting endlessly repelled even though on paper the odds looked good.