What is the result of a contest station not submitting a log for the contest? Are all the contacts nullified for the other stations that contacted him? Is it bad form for a non-participating station to contact stations that are participating in a contest?
In general, it is not a bad thing to work stations in a contest and not submit a log. In the big popular contests that don't have difficult exchanges, such as the ARRL DX contest or the CQ WPX contest, the majority of participants are casual. Casual operators are not trying to beat their buddies, or their scores from the year before; they are just there to hand out a few QSOs just for fun, and maybe pick up a new state, province, or DX entity. The vast majority of casual entrants don't bother to submit their logs.
When the time comes to judge the contest, the scoring "robot" (computer program) must judge whether a given contact was completed accurately. If both stations submitted their logs, and the call signs, frequencies, and times match, and the exchanges sent match the exchanges received, then the contact is considered good (assuming that suspicious behavior patterns that might indicate possible cheating are absent).
Things get more complicated for the scoring robot if only one station has logged the QSO. The robot then looks to see if any other stations logged that station at the same frequency and time. Often, the call sign has been miscopied; if W1ABC logged VE2XYZ, but VE2XYY submitted a log claiming a QSO at that time with W1ABC, then W1ABC will probably be judged to have made a mistake copying VE2XYY's call sign.
Consider the situation in which several stations log a particular call sign, for example K9EFG. If eight stations reported working K9EFG, but K9EFG didn't submit a log, the most likely reason is that K9EFG worked several stations but didn't submit a log. Almost all contest robots will give the eight stations the points for having worked K9EFG.
Most of the big prestigious contests email reports to stations that submitted logs after the contest has been scored. The reports give the station's final score, but also an explanation of points that were subtracted for mistakes, and often even a report of stations that miscopied the station's call sign or exchange. These reports make excellent feedback, letting the operators know exactly what mistakes they made. CQ Magazine call their report the "UBN" report, for Unique stations, Bad call signs, and Not-in-log reports. Of those three situations, a bad call sign or a not-in-log will definitely cost points.
But then there is the interesting situation in which a station logs a particular station that no other station logged; a "Unique" in the CQ Magazine UBN report. That could be caused by a casual contester making only one QSO, or it could be caused by the operator getting a call sign wrong. Most contest robots will give the operator the benefit of the doubt for a unique call sign, but it does look somewhat suspicious, which is why it is flagged in the UBN report.
My personal contest etiquette recommendation for casual contesters is to try and work at least two or three stations, so that your call sign won't be unique. But if you should make a single QSO and then be called away from the shack, don't worry about it; stations actively participating in the contest want your QSO, and are willing to accept the inevitable "uniques" in the UBN report to make QSOs.
On the other hand, if you make just a few QSOs in a contest and you are using contest logging software, then I recommend that you submit a log, for two reasons: first, it helps the contest organizers score the contest more accurately, and second, for major contests you will receive the report giving excellent feedback on where improvement is needed. Most contests give the option to submit a log as a "check log", which is used to check the accuracy of other logs, but does not get scored for points. So if you might submit a log but don't want to be embarrassed by being listed near the bottom of the results, then submit your log as a check log. I don't think stations that submit check logs get feedback reports though.