In this special case, you should sacrifice Resisting Spells for getting the others to 120. For the Mage part, you can deal out more damage and the Tamer part can control much more powerful pets.
Whether you generally need Resisting Spells or not is a strategic decision.
As a Tamer, your pet should be the one to take the damage - that's the main idea behind it. Experienced Tamers take care that their pets are always between them at the monsters. This is not easy though in a multiple spawn area but with quick moves and clever use of Invis spell it can be done. Just don't draw any attention on you and you should be fine without Resisting Spells skill. Either way, Wither and the other area spells remain dangerous for Tamers because they will make the monster switch target to you.
As a Mage-Tamer however, you surely want to deal out some additional damage - else high Magery or Eval Int would be a waste. In return, monsters will possibly change focus on you. When a mana drain, paralyse or poison spell hits you, your vision can get grey very fast because your ability to cast is reduced. In this case high Resisting Spell skill really helps to shrug off the negative effect.
If affected by this, set the pet on your attacker and run away, heal/invis and return to your pet. Protection can be an option to cast spells without interruption but it slows down casting rate. Don't use it if you cast Greater Heal frequently on your pet.
