There are good reasons to buy and use the hypenated version, but I would ALWAYS want the unhyphenated version as well. As stated before, some companies actually use the hyphen linke U-Haul who uses both versions for their website (u-haul.com and uhaul.com).
Another reason is that keywords are better detected by search engines and software with hyphens separating them. Sometimes it weird combos sound good until you see them without the hyphen. Let's say you have a website specializing in hit songs from the 50's and decide to call it 50's Hits dot com. 50s-hits.com has a totally different look to it than 50shits.com. Does the "s" belong with the 50 or the hits? Same thing for triple letters if you have a site called say Pass saver .com for example, many people will type passaver.com instead of passsaver.com since it looks like too many S's. However, pass-saver.com makes it easy to see the correct spelling.
For speculation, however, I generally stay away from hyphens, as they are ususally available to the end buyer of the unhyphenated version.
Buying the unhyphenated version of a generic in use hyphenated can capture some of the traffic, if it can be done without trademark or unfair use issues such as direct competition.