That's a good point: a lot of undead stuff keys off Cha (Concentration, most save DCs, rage duration etc.). Having Cha set to 1 is really going to hurt all of these.
The only conceivable role I can see for a Necromental PC is beatstick, without special attacks that have a DC, and wading into combat with mediocre to poor hit points.
As liquidformat and I have pointed out, a set Int of 1 is problematic for a PC, even more so than an Int of --. I think if you wanted this template to be usable for a PC, you might need to house rule the ability mods to Con --, Int -10, Wis +0, Cha -10 or something similar. Honestly I don't know why the devs didn't just make it Int -- and be done with it.
As written, to get to the 3 Int required for PCs, with a set Int of 1, the tow things that spring to mind are Headband of Intellect, or the Prodigy NPC trait (DMG 2). Otherwise an ally with two Wish spells handy.
Despite the very worthy purpose of these threads, some monsters/templates are just not mechanically suitable to be PCs.