no, you don't get a bonus for spells that carry across classes - they are just there to give people a bit more options.
A pure caster char has advantages in ridiculously high mana and monster regen, as well as access to a huge variety of spells - however, you are gonig to sacrafice hitpoints, and alot of your abiliity to solo stuff by making a char like that. For groupbunnies who never solo, that is fine, and probably a usefull thing (although even then, not having skills like protect and glance can be considered handicaps).
Basically, there is no 'best' classo - play around and find one that fits the style of play you, personally, enjoy. Most of us who have been around for a few years (or quite a few years, in the case of some people) you will notice have 2-3 chars that we play, generally with very different classos - some for cycling different kinds of mobs, others just because, say, a thief, is played very differently than a mage or a warrior, and can be alot of fun - but then is not suited for nostab mobs, etc.
Experiment, have fun, and don't be afraid to ask questions
