2,025,000 XP is what your characters will have at around the end of a normal game and I found I didn't need to cast as many spells late in the game either as my characters could kill the enemy without magical assistance, At that time, clerics were used to top up HP if the enemy landed any hits (which they didn't always). This is why I only compared the cleric and fighter/cleric at lower levels because with an optimized and well armed party, cleric spells aren't that relevant late in the game and the ones that are are can still handled well enough by the multiclass cleric. What you said may be true in HOF mode but the OP is playing through on normal difficulty and on normal mode, single class clerics ain't that great.

Turn undead can be good or bad. It was useful in cramped areas to get a little breathing space but I never used the ability all that much. It was also annoying having to chase down undead that fled into unexplored or trapped territory. I thought bottlenecking them in a doorway with 2-3 tanks in the front was much more effective than turning undead and have them run away.

All characters should be doing damage or at least causing some trouble for the enemy, the singing bard is the exception. They're very useful against the harpies in TOTLM and their combat bonus songs are always great and best of all, the songs don't have durations, spells do. Even when putting a cleric in a support role I still say the single class cleric doesn't cut it. The extra few clerics levels did made a difference in my games but the differences weren't in my favour. When I switched the single class cleric out for a multi class, I was getting better results.