...can you spread over your characters skills before your character becomes diluted to the point of uselessness? Just curious, since I've been messing around with a few different skills on alts just to see how they benefit how I play the game. For example, could I make a mage/thief with perks heavily invested in conjuration, alteration, destruction, sneak, plus one handed weapons and still find the character useful in combat or would this Jack-of-all-Trades style just make things increasingly difficulty?
Conjuration, Alteration, and Destruction with a minor in Sneak isn't an uncommon build among pure Mages, and it works because each perk tree fulfills a separate important function: Conjuration provides crowd control; Alteration provides defense; Destruction provides offense; and Sneak is great for all-around utility.
The danger of spreading yourself too thin comes most often from trying to heavily perk two different trees with the same basic function. For instance, why do you need One-Handed weapon perks for this build? Conjuration and Destruction provide a lot of fire power already. If you want a melee option, take the backstab perks in Sneak; you can effectively use daggers with a minimal perk investment that way.