I was talking in the hall a few days ago to 2 of my colleagues, one tenured and one not about the issue of going up early. My tenured colleague asked "why would you want to?". My untenured colleague said "I totally would if I could."
On a personal level, I would like to have the certainty of tenure. We moved to Prodigal City, which is far from National Lab City, because I am pretty sure I will get tenure here. I have no desire to uproot my family again after 6 years, especially since the Prodigal kids will be in school by then. Going up early means fewer years of tentative roots just in case. Professionally, I'd get a 5-6% raise (which is nice, but not really a motivator). When recruiting, I'd be able to tell students I have tenure (which can be a big deal to some students).
So what are the costs to early tenure? It is true that in my department assistant professors are somewhat protected from onerous service obligations, so I'd be giving that up. I would be giving up eligibility for "young investigator" and "new investgator" awards, but I am already not eligible for any of the shiny Federal young investigator awards anyway, since I am too far out from earning my PhD (which is totally fair, since I have a lot more experience than a new grad). If I am denied early, that could put a black mark on me for future rounds.
In the short term, going up after 3 or 4 years won't really change what I am doing. I feel like I had a great first year, and would like to capitalize on that momentum. If I can get some more funding next year, maybe it becomes worth it to push for tenure after 3 years in case I have a dry spell later. So, what would you do?