I do agree that having church donation systems be something other than a gold sink is nice, but I don't think that it is limited to clerics (ie spellpraying investors), because there are many ways to be devout...and more importantly, it would be good to establish some perks and penalties to those wishing to be in service to a god, whether a cleric or not.
People that actively choose to walk the devout path in general will have a pretty hard time of it, with no real reward for their service save the cool roleplay that can come from arguments and occasionally traveling with a cleric that can raise them for free. However, there is also no real penalty, no real reason "not" to follow a deity, except that you can avoid the dogmatic drivel and avoid meeting enemy clerics that can't raise you at all. For anything meaningful you have to either be a cleric or be traveling with a cleric and thus feed off of their interaction
Either that, or follow a rare god and be really, really lucky on a quest!
So having something more ... maybe not tangible, but at least more inclusive to all members of a faith if they are devout, including anything that comes of the tithing system, seems like it would both a) give more reason to be involved and supportive of a faith you have chosen, and b) give more reason to pick no deity at all rather than just filling in fields, as well as leaving the inactive and apathetic in the lip service category.
Also, to some degree I would agree with 8bit and Honora... I don't think that donating should necessarily equate to reward, at least not directly from deity... but it would be nice if there was a piety system in which tithing
contributed. By performing acts of the faithful, one collects a certain amount for, say, stronger blessing effects on your person, or maybe even as the original poster suggested, a sort of divine favor or "Prayer" ability that all faithful have, but only the devout can ever make use of. Rather than being instantly jumped on by DMs for buddying up with an enemy, you could simply be slowly losing those 'piety points,' whether in great or small numbers. At one point you could even fall from grace because of it, or in the case of the non-divine characters, have the deity quietly dropped from the field. If a cleric lost his deity, they would not be able to cast spells anymore. DMs that witness acts of the faithful (or unfaithful as the case may be) could add or remove such points manually.
Other than being included in the ways to be considered devout (a minor one, even, as after all, heretics and murderers can donate to a Toranite church) it seems that donations could just determine what services a particular temple could provide. After x amount of donations, they could offer better healing potions for sale, or faith based trinkets to the faithful appropriate to the deity. Housing for paladins (in the sense of a small room for sleep and prayer with a single chest), that sort of thing. In addition, if you have contributed a great deal to the church, there should be no problem in getting a blessing or remove disease for free every now and then if you don't have the funds. Money doesn't equate to faith, but it would be cool to have it actually help your temples. I think I'm kind of rambling, but you get me...perks and penalties, the latter of which I haven't delved much into but should be present. From those given much, much is expected, right?