1) Sleep deprivation has the equivalent effect on the body as alcohol. Anyone who's stayed up all night studying or playing dodge-ball in a church gym can testify to the fact at some point in the night you just feel different. One study I heard of said being awake for 18 hours without rest is the same as having a 0.05 blood alcohol level. Sleep deprivation isn't healthy for the body, so why promote it and disrupt that which God gave us to enjoy, sleep?
2) Volunteer Shortages - The key problem in most church youth ministries is the volunteer problem. Many youth pastors have to cut back on programming because it's too hard to find people to staff them. Lock-ins are no exception, because what right-minded adult would forsake a warm bed to eat cold pizza and walk around a church on Make-Out Patrol?
3) Spiritual Significance - This is what I ask about any program, event, or activity: What is this doing to glorify God, point to Jesus, lead the lost to Jesus, and help us grow in our faith? Too often lock-ins are parties with a Bible passage in the middle. If you're going to have a captive audience for 8-9 hours, don't waste the time! It doesn't have to be a serious reading of Scripture all night, but if the purpose of the lock-in is just to have one, you're wasting your time, their time, and the church's money
4) Exhausting - The present-day youth pastor is often pulled and strained in so many directions and live in a constant state of exhaustion. Gone are the days when it was a special moment to have your youth pastor come to a ball game or event or whatever, now in many places it's an expectation (again, find that in Scripture, but then again, most job descriptions aren't biblical either). That said, a lock-in that requires 30-40 hours of preparation, recruiting, set-up, and execution is going to cause a youth pastor to choose between neglecting his wife/family or the necessary time he needs to study to teach.
5) Dark areas, teenagers, two genders, and hormones a-raging. Need I say anything more?
I'm not saying people who do lock-ins are bad or that their ministry isn't godly. All I'm saying by this is why I don't like them and don't plan on doing them anymore. They are great once in a LONG while I suppose, but to regularly do them is asking for rapid burnout.
These questions should always be during planning - What is this event/program/speaker/retreat/etc. doing to the glory of God and the health of the local church? Will this present the Gospel accurately and provide an environment for the Spirit to work on the hearts of those present? Will Jesus or You shine because of this? How is this encouraging missional living among everyone's circles of influence?