Because of their lack of armor restrictions, Clerics make for decent front line fighters with enough Con.
Frankly, Clerics fulfill a role and that role is healing and support. If you're not doing or making the best of a cleric's spell casting, then what's the point?
Other classes are meant for front line fighting. Paladins hardly have the Cleric's range of spells but they do healing better than anyone with Lay On Hands and have low-level support spells like bless, etc.
Dual/multiclassing spell casters prohibits your top tier spells. For clerics, having an extra +1 to weapon proficiency, a marginally improved THAC0 and a bit more HP isn't worth it.
Just give em a morningstar and some plate mail and let em go to town.
Still, I suppose a few levels of fighter can't hurt if you *really* need that extra kick-ass potential for some reason.
Paladin's Lay on Hands is only usable once per day and is not as strong as the Cleric's Heal. In fact, I would argue that, at high levels, especially once HLAs come into play, that Mages are better healers than Paladins (just look at Wish and Summon (Fallen) Planetar, two of the best healing spells in the game, and remember that Project Image exists as well (try it with a Rod of Resurrection)).
Dual classing, if done early enough, does not cause you to lose top tier spells of your target class. Specifically, dual classing at level 9 will only cost you about one level in the long run (considering BG2 and its expansion), and dual classing earlier will cost you even less.
Cleric actually synergizes nicely with other class, thanks to the Draw Upon Holy Might spell. A Fighter/Cleric can use DUHM to get more Str and Con, allowing her to deal more damage in melee combat (and the Dex bonus can help with AC as well); getting your Str to 19 this way provides a huge bonus (who cares about exceptional Strength when you can get 19 Strength pretty easily?). For a Cleric/Thief, many Thief skills depend on Dex, and DUHM will boost that stat; casting DUHM will make her a better thief. Also, some thief skills can be used without breaking Sanctuary.
Cleric/Mage synergies don't really exist in BG1 (unless you are playing EE or a BG1-in-BG2 mod and manage to get ahold of a Minor Sequencer scroll). In BG2, however, one Minor Sequencer scroll and you can now instant-cast those slow Cleric spells like Bless and DUHM. This gets even better when you reach high levels and get better Sequencer and Contingency spells; that class really shines if you manage to get 6 million XP. Fighter/Cleric/Mage, while unable to get 9th level Mage spells even in ToB, can still put DUHM and other Cleric spells in sequencers for quick casting, cast a sequencer, and then be effective in melee, with Stoneskin to negate damage (especially useful once you're at the point where AC becomes useless), and Protection from Magic Weapons for better protection at higher levels.
Remember that having the Fighter class gives you extra attacks that you wouldn't otherwise get.
"My day" started with the D&D boxed sets through to 2nd Edition, plus the SSI games. [They really do not stand the test of time, unfortunately].
Actually, I would argue that the SSI games had problems even then. The Gold Box games were not fair to female characters (they had reduced Str caps and nothing to compensate); this sexism made the game less attractive to female players (I happened to read a contemporary review by a female gamer who didn't like Pool of Radiance, and that was one thing she didn't like about the game (especially since other games (Ultima, Bard's Tale 3, and Might and Magic) allowed female characters without any such handicaps. Fortunately, this rule was dropped in 2nd edition AD&D, and the non-Gold Box SSI games do not implement such a rule.
There's also the whole issue of racial level caps. The racial level caps were basically the designer's way of saying "demi-humans are useless at mid-high levels". This means that, while Pools of Darkness offers many interesting options for multiclassing demi-humans, they're all completely useless, as they are capped at lower levels than single class characters start at. (Have fun with a level 5 Half-Elf Cleric in a game designed for characters to grow to level 40!) It is a good thing that the Infinity Engine games (especially BG2; can you imagine Throne of Bhaal with racial level limits?) do not implement those limits, and I believe this was a completely intentional decision, along with not implementing stat-based spellcasting level limits and not implementing permanent stat loss from revival spells.
Also, from what I understand, Pool of Radiance made healing painful by only giving you one weak healing spell for the entire game and not having any effective shortcuts for healing (and resting heals almost nothing). Even Wizardry gave you slightly better healing spells at mid levels and MADI (equivalent to AD&D 2e/3e Heal) at higher levels. Ultima 3 has healing fountains, and Might and Magic heals you fully when you rest (and there's Power Cure and Divine Intervention).