Crystal does help with head retention, and gives some residual sweetness to beers. Depending on the type of crystal it can also give Honey-ish, caramel-y or fruitcake-y flavours and aromas as well.
Crystal can also help balance out big hop bitterness (in all-grain this can be accomplished by mashing warmer, leaving more unfermentable sugars in the wort. Perhaps the inability to do this with extract brews is one of the reasons crystal seems so ubiquitous in extract recipes.)
While a large proportion of ales do include crystal in the grain bill, there are plenty of good (great even) beers made without crystal malt. For example, many German styles (e.g. Pils, Alt, Hefeweizen) use little or none, and lots of people around here make fabulous SMaSH ales using just Maris Otter, or just Vienna malts.
I don't see any real reason NOT to use crystal malt, but if you want to I'd suggest one of two courses of action:
1. Find recipes that don't use any crystal
or
2. Find recipes with crystal, but understand what it's doing there and how you'll be changing the beer by leaving it out
That way you'll almost be certain to be pleased with your results.
Happy Brewing!