I assumed corporations were relevant as much of this discussion has been saying unions should be banned for the public sector, implying that they're acceptable in the private sector. It's also where unions started.
Anyway.
At my previous job, in the British public sector, there were three unions who represented the staff, and I joined the one the others in my office had joined. Initially, this was because I appreciated the support a union had given someone in my family against a bad manager. Later, I stayed in the union in support of those with much less skill than me — the union was campaigning against "zero hours" contracts¹.
A year or two ago, the government imposed significant cuts. (Contrary to a lot of posts in this thread, government budget for salaries isn't unlimited.) I found out that my union seemed to be prolonging the negotiations unnecessarily, by being needlessly difficult — submitting 1000-page documents in the last hour of a 3-month consultation, for example, which had no more content than a 5 page document on the first day. Most staff I spoke to were more interested in knowing whether their job was ending, rather than hanging on for an extra month's pay during negotiations. (We already had 1-3 month notice periods.) I left the union, and didn't join another. Several colleagues left, and joined the more reasonable union.
For another example, the RMT is the more extreme union representing London Underground employees, ASLEF is more moderate (there are others, these are the two biggest, and not all staff are in a union).
Quoting from [2], just as an example, "The RMT has attacked a fellow Tube union for failing to call strike action as part of the ongoing Night Tube dispute. The RMT accused ASLEF of being "happy" with alleged assurances that London Underground (LU) would put forward new work rosters for drivers." And just look at the language in the RMT newsletter "only resolute determination will prevail...".
RMT goes on strike much more often, and make a lot of noise about it, but most of the staff in that union are less essential (ticket checking, platform staff, etc) and the trains keep on running. The train drivers generally belong to ASLEF, which is still a powerful union, but rarely tries to defend drunk drivers or similar.
[1] https://en.wikipedia.org/wiki/Zero-hour_contract#United_King...
[2] http://www.ibtimes.co.uk/tube-strikes-rmt-attacks-train-driv...