I would respectfully disagree with my colleague's response.
While I cannot comment on the treatment of Prelims at other programs, the Prelims at our program have the exact same rotations (ie, speaking on non-designated Prelims) as the Categoricals, same call schedule, and are in the OR the same amount of time aS the Categoricals. Frankly, I don't think the attendings make a distinction nor do the residents.
I agree that by taking a Prelim position you are risking being out of a job as a PGY2, given that the number of open Categorical positions in the scramble is exceedingly low and the OP wishes to avoid the scramble, wouldn't it be preferable to have a position for a year, rather than sit out the match?
Again, while I cannot speak for other programs, we have long accomodated Prelims who have worked hard and expressed an interest in staying on as a Categorical. Many programs with residents going in and out of the lab can do so, as there may be positions opening up in the 3rd year (this would necessitate being a Prelim for 2 years). We've tried to be up front with residents in such position - I suppose you could find yourself in a program where you'll be "promised" consideration and then end up jobless after one or two years, but I'd hope that most PDs would be honorable and really try to find you a Categorical spot if you do well during your Prelim year(s). Thus, I don't think its a guarantee that you'll be jobless or even have to leave your program after the first year. A good PD will assist you in finding a PGY2 position, even if it means at another program.
The trick is that you must rank all Categorical positions you might desire before any Prelim positions on your Rank List. This way you will match to a Categorical position before a Prelim one. Caveat: do not rank a position at a program you wouldn't be happy at simply because its a Categorical one.
Finally, we get candidates every year who rank us both for a Categorical and a Prelim program. We realize that getting a Categorical position is growing more and more difficult and don't see this as a lack of interest, but rather a sincere desire to find a position at a program they like.