Synthetic is great to look after, they're durable, you can take them to the beach and just hose them off afterwards, they're that easy!!
Yes they do also have quite a long life span - HOWEVER, a good quality, well looked after leather saddle will outlive a synthetic.
In saying that synthetics are good, I must confess that I am an avid leather fan and wouldn't buy a non-leather dressage saddle now, especially for competing. I do have a Wintec GP for beach riding and 'bush bashing', and it's comfortable for that, but I still find my leather saddles far nicer to ride in.