Chickenpox is generally a mild virus in children (which is not the same as saying it won't make them feel very ill and extremely itchy, albeit that in some children it doesn't even do that, or that it is over and done with in just a few days, just that it is very rare that it causes complications). Something like 90% of people in this country get chickenpox as children - most commonly between the ages of 2 and 8.
I would say it might be worth getting the vaccine if:
your child gets bad eczema or asthma, as blisters mixed with itchy painful eczema probably would increase the risk of getting infected spots (and therefore make scarring more likely - something that is in fact very rare with chickenpox, albeit some of the scabs may leave a very faint mark on the skin for up to a year afterwards), and one of the extremely rare complications of chickenpox in children (much more common in adults who get chickenpox) is pneumonia, which would obviously not be good combined with asthma;
you are planning a once-in-a-lifetime holiday that you wouldn't want ruined by a case of chickenpox and are willing to pay the over-inflated price for the vaccine to avoid the risk (when I looked into the vaccine for this reason, I decided to take the risk of my children getting chickenpox, because I thought the price per vaccine charged at the nearest place to us that would provide it was excessive - about double the cost of any of the other vaccines on offer);
if you or your husband had chickenpox with complications as children (you are quite likely to have had complications if you got it as a teenager or adult) and are worried this increases the chances of this happening to your child;
you have never had chickenpox and don't want to take the risk of getting it as an adult, as it is NOT a mild illness in most adults.
The main disadvantage is the cost (particularly since it is so very unlikely to be anything other than a mild illness if caught in childhood). However, it is currently under consideration to add the chickenpox vaccination to the list of routine vaccinations in this country, so the main disadvantage of it may soon no longer be a disadvantage.