I've seen it in many posts, an advice to get a character all the good/necessary gear before doing TR? What is the reason for this? After TR you have to grind much more xp, so is it not better to TR as soon as possible, and then grind gear and xp at the same time?

I've a monk at lvl20, I'm a new player, it's my first lvl20 char. Also have a paladin close to 20. I'm thinking about TR'ing my monk, mainly for Past Life feat - seems too good to not take it. I don't see a need to grind for any uber gear right now - after all I got to 20 with no problems with the current gear. Why would I want to grind any items now, losing all the xp while at it, since I'm capped?