People leave their churches for all sorts of reasons and in all kinds of conditions. Some leave churches wandering out of a fog bewildered, some surf the edge of the blast wave after a big blow-up, some leave at the end of the left foot of fellowship, and some lose motivation or faith and fall off more than they depart. Some leave because they find another place more attractive, and some just want something new. Everyone that leaves has their reasons, I'm sure.
Although I doubt that many are legitimately motivated when they choose to leave a church, I do think that leaving a church can be the right thing to do.
If that church doesn't uphold the Scripture as the infallible rule of faith and conduct;
If that church embraces universalism;
If that church becomes libertine or antinomian;
If that church adopts legalism...You get the point. There are practical and doctrinal issues that are so fundamental and non-negotiable, that if a line is crossed there, then we must cross ourselves off the roll. Even if this is the case, I don't think one should leave such a church without a fight. Not that one should seek to win an argument or engage in a turf war, but that one should contend for the faith and for the souls in that body. Don't let them wander off to hell without an effort to save their souls! However, if they won't hear, and won't stand on sound doctrine, then one must leave!
At times, a bone of contention arises between folk that, given the nature of the personalities involved, cannot be resolved. If continuing on together in mission is impossible, separating unto mission is acceptable. It is still unfortunate in the grand scheme, but as long as it is done on reasonable terms and doesn't result in an unending grudge it may be the preferable course of action. We can disagree without being disagreeable, even if it means one going one way and the other going another.
At times, folk are being appointed in the body according to the wishes of the Spirit of God, and leaving one congregation and going to another is precisely what God wants! It's easy to discern this if one is moved to a distant place; it's not so easy if this change takes place in the same town. Regardless, each of us is a gift to the body and we must understand that God gets to place us where he wishes. Actually, I wonder how much dissatisfaction people feel in church is actually just the dissonance in their souls caused by not discerning where God wants them.
There are acceptable, justifiable, and quite spiritual reasons to leave one church and go to another.
And then there are reasons which are neither expedient nor justifiable.
It is not justifiable to leave a church for selfish reasons. Church is about Jesus being Lord, not about the churchgoer getting what he or she wants. Christians are not customers, the church is not a business and spiritual ministrations are not consumer goods. To treat this God ordained endeavor as if any of that were true is an insult to grace. Leaving a church for greener pastures is unacceptable for clergy or laity.
It is not expedient for those who have been appropriately corrected, or who have been properly spiritually directed to leave a church rather than humbly submitting to that which has been rendered for their spiritual development. The flawed natural constitution of humans beings means that we grow as Christians to our lowest level of incorrigibility, but that cannot excuse a lack of obedience to the Word or to the brethren. Escaping correction or rejecting direction in one body doesn't give one a blank slate to start in another (regardless of whether one is clergy or laity).
There are occasions where the godly will be justified in leaving a church. At times it will be the absolutely right thing to do. However, even if it is, it's never something left merely to our discretion or preference. Jesus is head over the body, so he gets to plant us where he wants.
As for us, we need to stay where we're planted and blossom.