-blank frames will positively definitely have less resale value when sold, but how much will it be worth to you now ?
i've seen countless 'mystery builder but definitely 531' and 'i think some guy told me it was a colnago' type frames go for way less than they would've if their provenance was known (even with damaged transfers)... actually this is a pretty good way to get a good quality frame for a bargain price..
-consider the value of having a versatile bike... down the road, when you find a frame you like better than this one, maybe it will be valuable to you to have the potential to convert it BACK into a road bike..
-i've found rust to be a really minor problem... my px10 had terribel (ie nonexistant) paint and it never rusted.. i was careful to clean it up after riding in the rain and i usually switched to another ride in the salty winter...

-lastly, consider where the bike will go when it's no longer your's.... some guys operate under the ideal that they don't exactly own their bike but are instead holding on to it until it's time to pass it on to the next guy...
a good steel frame could last a very long time...