A skimmer isn't a 100% necessity. It can make things much easier for you and your tank's inhabitants, in addition to keeping things cleaner, though. Also, in many cases, some skimming is better than none. However, that kind of attitude isn't necessarily conducive to the overall thickness of your wallet, so to speak. You'll end up spending less money in the long run by buying a good skimmer from the start, rather than overpaying for a bad one only to buy a good one later--spending more money overall.