Why should I max skills?

The argument usually goes: Skills cost more when they are at a higher level, and most of the time, they don't give the same mana/damage efficiency as lower levels. So why max skills at all?

The simple answer is when at a higher level, players generally make much more money. At level 10, 10,000 Mesos is doing well. At level 60, weapons may cost as much as 15 million Mesos.

The money "wasted" from maxing out skills is minuscule when compared to the money used on equipments, scrolls, and so forth. You will always make more money, but you cannot always do more damage.

And this goes for 'all mana- or HP-based skills, including Slash Blast, Double Shot, Magic Claw, Lucky 7, etc.

More importantly, when maxing out a damage skill, you save time. 2 level 10 Slash Blasts may do more damage than a level 20 Slash Blast, but a level 20 Slash Blast saves time, especially since that level 20 Slash Blast has a much better chance of a one-hit kill than a level 10 Slash Blast, and costs less MP.
http://strategywiki.org/wiki/MapleStory/FAQs