not if they are licensed. Of course if they are not and you get visited by an inspector....ooops.
Installing cabinets, you can use an unlicensed person depending on where you are I suppose, but any electrical or plumbing is supposed to be done using licensed individuals, UNLESS (in some areas) you are going to do the work yourself.
It's like anything else, you have to make a judgement call about who you use. I never call large companies to do work, I use electricians and plumbers who work for themselves or guys who do side jobs.
It sounds like you talked to some General Contractors who gave you bids doing and entire job. That can be a very expensive way to go. You will basically be acting as your own General Contractor if you partition the job and do your own hiring and scheduling. That will save you a lot of money but the headaches can be painful, but it's important to not let guys not showing and making mistakes freak you out. If you are a pretty calm person and can tolerate a lot, this approach is fine. Be careful about how you pay this kind of help, get it in writing, and don't be up tight with them.
You might ask them to supply thier license number on the contract, and for a few references.