Rolling an old 401k into a new 401k is generally only for ease of management. For example, how many bank accounts do you really want? As long as the funds are reasonably allocated I've found it can be a useful "mind game" to leave it separate. Sometimes it's desirable to ignore an account and let it grow, and it is a nice surprise when finally adding all the account balances together. In other words, I keep thinking I've got X (the amount of my biggest or current 401k), which affects/helps my habits and desire to save. When I add them all together I'm shocked to find out I've got Y (the total of all accounts).
Personally, I've had big paperwork problems transferring an old 403b (same type process as 401k) even when I had an adviser helping me move it. In the end it was worth moving it, because I'm having the adviser manage it.
I'm actually writing this answer specifically because I recently moved a big 401k into a Traditional IRA. A rep from the brokerage, representing my previous employer, kept calling me to find out how they could help (I didn't brush him off). I found that using an IRA provided me with the opportunity to do self-guided investments in funds or even individual stocks, well beyond the limited selection of the old company's 401k. It was useful/interesting to me to invest in low-fee vehicles such as index funds (ie: the Buffett recommendation), and I'll find some stocks as well. Oh and when the old company 401k has certain funds being discontinued, I didn't want to notice the mandated changes years later.
So, I'd suggest you consider management and flexibility of the 401k or equivalent, and any of your special personal circumstances/goals. If you end up with a few retirement accounts, I suggest you use an account aggregating website to see or follow your net worth. I know many who, based on various concerns and their portfolio, find an acceptable website to use.