As with everything, there are tradeoff. With wider gears, you give up quickness and precision in shifting, plus you have bigger jumps between cogs and more weight. A wide range cassette requires a derailleur with a long cage and with the jockey wheels positioned farther from the cogs, thus slower and less precise shifting. Though with the 9 and 10 speed cassettes these days, having a couple low gears you seldom use isn't much of a hardship (I still use a 6-speed freewheel) unless your shifting is affected.