By all means yes get a handheld pinpointer, though you don't actually need to.
You'll be glad you did. Maybe you would be better to start out a week or two without one like I did, then you'll know what your missing.
The 250 pinpoints very well, but at times you might be off half an inch or so - And thats enough for you to dig a 10 inch hole, when in reality the target is only 2 inches down in the side of the hole. A pinpointer wil ltell you right off the bat if you've made a simple mistake like this and also as another has mentioned reduces your odds of scratching up your finds.
Besides scanning holes they are also great at scanning the dirt piles to see if the target has alreay been removed from the hole.
Granted you can of course do this with the detector itself, but its much more conveient to use a handheld.
I wouldn't try to take business away from kellyco they are a great outfit and in fact I got my ace 250 from them, but I think their pinpointers are overpriced. Check out the wizard 2 that melcrose posted. Don't know about the little wiz, but the big gun shaped one is a kick a$$ instrument, with adjustable sensitivity, vibrate/audio/visual alerts, a detection range of about an inch and a half for a penny on max, and dang close to nothing on minimum - And even a LED spotlight for those dark holes which actually comes in handy now and then.
Be aware that whatever pinpointer you decide on, they are fragile instuments and their long PVC necks are prone to snapping rather easily, so resist the temptation to aggresively probe with them.
I snapped mine twice but was able to patch it back up with some JB weld.
As far as bulk/inconvienance in carrying, I got me one of those retractable metal key holder things that extend a line out about a foot from a hardware store for 99 cents.
It is only about as big as a silver dollar and actually not much thicker, and it clips right on your belt/pants. I just hooked up my probe to that thing and its always there hanging by my side ready to use. The weight of the pinpointer keeps it from fully retracting, which is fine. It hangs right above the knees to the side and I hardly know its there, yet I can whip it out right into a hole or whatever when needed, then just let go of it and stand up and it snaps right back into place.