MP management is the primary issue WHM have in end game.

Here are a few scenarios where MP becomes an issue:
1) WHM is doing DPS
2) WHM is spamming heals or overhealing
3) Raid group is taking unnecessary damage
4) Co-healer is not pulling their weight
5) Tanks fail to use cool downs

The first two points are areas where the WHM must work on. There's no reason a WHM needs to DPS. Improper use of spells and buffs is another issue WHM's run out of MP and you should NEVER overheal. Heal timing is also critical. Know when to heal and when not to heal.

The last three points are usually the reasons why a lot of people who play WHM complain about MP and fail to recognise what is going on. If the party as a whole is doing something that is forcing you to burn MP then it is your responsibility to inform them what is happening. This will allow your party to adjust things accordingly.

Find an equilibrium between heals and MP for each fight and your MP worries are gone.