By itself, a leaking main or intermediate shaft seal will not cause any failure of the engine... as long as the leak is caused by only the seals and it doesn't suddenly get worse while driving it. Keep an eye on it and keep it topped off with oil.
Having said that, I've never heard of one of those seals suddenly blowing out and dumping the oil quickly, and I've never heard of the bolts supporting the end of the IMS coming loose and leaking like a IMS seal or RMS. If one of those things happened, continuing to drive it could be bad. Also, if it started leaking badly enough I suppose the oil could foul your clutch, but you are (and should be) replacing it when you fix the leak anyway.
If it were me I'd probably drive it for a while, keep a close eye on it, and start shopping for a mechanic and parts. Oh, and buy a bag of Oil-Dri.
