Because there are other forwards who can play defense who bring more offensively too. One dimensional is one dimensional, it doesn't matter if the guy is all offense and no defense like Leino, or all defense and no offense like Glendening. The comparisons to Matlby and McCarty are nice, but not accurate.
That's 12 forwards better than Glendening. Andersson is also more skilled and good defensively, although Glendening is far more aggressive and adds something that Andersson doesn't, but so does Callahan, and Callahan has shown better hands than Glendening. I'd rather see Andersson and Callahan as the reserve forwards, and Glendening sent to GR as a call up.
I'm still failing to see the issue with Glendening. He's excelling as a shutdown center and agitator, he's been generating offense and chances, and now he's finally potted one. Callahan's a winger, and I tend not to put much stock in how good a fourth-liner's hands are. If we were absolutely starved for goals, I'd be worried. But we have firepower up front, even without Z.
I suppose this is sort of moot, though, because I mostly agree with your lineup. Indeed, I've found it pretty much impossible to squeeze any one of Andersson/Glendening/Callahan into all the different "ideal" lines I've been toying with. I've wanted Jurco on the top line with Z and D for a long while now; I believe our current top line would be dangerous as a second line; I'd have Weiss on the wing on the third line; and I'd want all of Helm, Sheahan, Tatar, Miller, and Abdelkader on the roster. And, I really do want Alfredsson back. I might swap in Alfredsson for Abdelkader, who hasn't impressed me this season.
I might hesitate to break up the kid line - but now I'm really straying from the topic. Glendening is awesome.