42 point buy? Wow.
At the risk of reviving the age-old "Is the MyTh over/underpowered?" discussion, the answer is: It depends. What's your party make-up? IMHO, if you already have a primary caster, your role (Mr. Buffmaster) will be rather boring. If you don't have either a cleric or wizard in the party, then an MT is practically a requirement.
In any case, the big hurts to an MT with Practiced Spellcaster (which is a must-have feat) are a) the reduced number of feats and b) the loss of highest-level spells. A Wiz3/Clr3/MT 10 gets three fewer feats compared to a straight wizard (four if you factor in Practiced Spellcaster) and can't cast 8th-level spells. He also has 1-2 7th-level spells, compared to the Wiz16's 3-4, and no familiar (if that matters). In return, he gets 13 levels of cleric spellcasting, meaning that he has a vast number of utility spells.
If your expected role is to buff the party and provide auxiliary healing (thus freeing up the straight cleric to be a combat machine), I'd say it's a worthwhile trade. If your goal is to engage foes in direct combat: No way. Assuming that a primary caster lobs all of his highest-level spells in direct combats, you can engage in such combats no more often than a 16th-level single-classed caster can, as you have roughly the same number of 6th-8th-level slots available. Moreover, you don't have the same number of feats to use on fancy metamagic, FWIW. Your real asset is the diversity of spells at your command, but in D&D combat, it's the specialists who are the true combat powerhouses, not the generalists. You can drastically reduce the percentage resources your party spends on a given encounter, which is an effective way of contributing, but you will not be a front-line showboater.
After MT 10, BTW, you should probably go for archmage, although the requirements are awfully hard.