Yes it is important to renew your domain name even if you do not intend to continue to host your web and email any longer. Web site that has gone live for years likely to be spidered by search engines and may enjoy some listing on major search engines. More often than not, expired domain will be picked up and have it pointed to pages with ADs or others.
A friend of mine has registered a .com domain and host his web site for many years before deciding to stop hosting his site two years ago, not sure for what reason and did not renew his domain. Over those years, he has distributed a great number of business cards to his bussiness friends and asoociates bearing his company’s web address. On one fine day, one of his business associates called and asked him a question that really shocked him : “When did you change your company business from being a renovation specialist to a company offering XXX services ?”. Yes, someone has picked up his domain and have it pointed to a XXX site.
It is pretty cheap to maintain a domain name these days and so do remember to renew your domains :-).