Maybe it's a matter of taste, but some of us like pure plays where one can see what the bet is. Your "well, they'll have something else in the pipe one day" stories are way too fuzzy to value. It's like a VC investment without the insight into and control of the management. Of course, it sometimes works out well - aka lucky.
Comments:
1) As a general rule having a deep pipeline allows MUCH more rational decisions. And there is nothing like rational decisions (rather than defend the one or two items unto death) to improve the performance of a company.
2) Having a deep pipeline indicates some degree of conservativism and skill that I would suggest is indicative of management quality.
3) Having multiple items in the pipeline allows flexibility like that just shown by ZGEN. It also allows for a better bargaining position when trying to partner.
None of this is to say that single product companies can't be good - but it is just harder. FWIW I consider JP the best biotech CEO I know of (albeit with the weakness that it is the Joe Show - in contrast to ZGEN which has significant depth of top notch mgrs) - but his job is just tougher.