First of all, they are Seals not Glyphs. Not a big difference, but noteworthy.
Secondly, thats the value for one rune. With a full set of Seals, you get 270 gold in 20 minutes. Add in Quints, and you get 636 gold in 20 minutes. Add in the +2 passive from a gold generation item, the +1 from every minion death, then the +8 or so from actually using your gold generation item, and you have quite a bit of gold without ever taxing CS.
And lastly, you might want to actually look at the gold values in your games. In one of my recent 20 minute games, I was playing support (no Gold runes) - I ended 0/3/7, 20 cs, and 5.4k gold. My ADC was 2/1/3, 90 cs, and 6.7k gold. I was a mere 1.3k gold behind my ADC, who got comparable gold from combat and who got 70 more CS. If I took Gold per 10, theoretically I could be .7k behind my ADC if i was able to generate the same amount of gold (not saying I would since I rely on my current setup of combat stats to get assists and such, but if I played a defensive support or something). In another one of my support games, I ended a 35 minute game 2/7/9, 29 cs, 9,7k gold. My midlaner was 3/11/6, 159 cs, 11.6k gold. I was a mere 2k gold behind my midlaner who got 130 more CS than me with similar combat gold. That could have dropped to only 1k if I told Gold per 10 runes.
So no, 636 gold in 20 minutes doesnt seem like a lot, but its actually quite a bit. Its 2 kills worth of gold in 20 minutes. Its 30-35 CS, which is quite a lot to be ahead by 20 minutes. In a game where 1 kill can snowball a lane out of control, getting that gold passively in 10 minutes can be pretty huge. It doesnt have as big of an effect as it used to, when passive gold was lower and supports couldnt start with Gold Generation items, but they are far from useless.