The reason for 55 health is reduce damage from all attacks.
The monk skill protective spirit will explain it greatly..
55 health, prot spirit makes it so u cant lose more than 10% of
your max health, so with 55, you only lose 5 health per attack

Goods:
1. can kill all melee things
2. great for farming
3. bragging rights
Bads:
1. mesmers are the way to kill a 55 monk
2. costly equipment (about 40k)
3. can die if attacked by too many, too fast
4. Ritualists wont die with this build usually, because they hold the urns and dont attack you.

Summary:
a 55 monk is a good choice if you are expirenced and feel like just getting money and fast. i would recommend making a 55 monk once you've already finished the game with a different character.

Tips:
get a 2nd set of armor that doesnt have all the superior runes in it so you can heal people if your group needs a healer.