A few problems with references are that employers won't give a bad one unless you really hack them off, which could be for something like leaving so they have to find and train someone else up.
Likewise they might give a good reference to get someone out of the way.
Even if references were any kind of indication of someone's workplace competence, in this line of work it seems to depend on when someone starts on the site - the first to get there know the site best so take charge, while those in later tend to end up 'helping', making them seem less competent, especially if they're not given adequate direction. The person responsible for giving direction will usually be the person asked for a reference; they're effectively giving a reference on their own management skills, not on the individual worker.