Meaning of merit point upgrade value?

[cheyrn]

For HP, a web page says +10 means +10 HP. What does +1 elemental magic mean? Is it actually +1. So, for example instead of 300 I would have 301? This seems almost worthless. Or, does it mean something else?

[Tidis]

each merit gives 2 skill, so if you were 300, you'd now be 302.

[Shirai]

Apart from that, each merit point option has a certain cap.
While you're looking at the amount given by a single merit increase you should be looking at the end total.

As Tidis above me sais, each skill merit ups your maximum skill by 2, which by itself isn't much but you can spend up to 8 merits in each total giving you +16 skill.
Information about Merit points: http://wiki.ffxiclopedia.org/wiki/Category%3AMerit_Points