it seems to me to be that if you leave, then there is nobody that can modify you code if the need arises

I think that any even vaguely competent programmer should be able to pick up the rudiments of a new language in no time at all. I don't think lack of familiarity with a language should be any stumbling block at all.

However, if the basic tenets of JSPs are wildly different than those of portlets then that might be an issue, but I can't see how it would really be.

It also seems that the other developers' aversion to portlets makes their knowledge of it much less significant.