i am inclined to agree on all points, and while the second parenthetical would be useful in Contemporary English, i cannot help but note that "þ" and "ð" in Old English both represented the same phoneme (unlike in, say, Icelandic). all of the OE fricatives (/s/, /f/, /þ/) had predictable phonetic voicing depending on environment (voiced intervocalically, unvoiced elsewhere), but no phonemic distinction. in extant MSS, <þ> and <ð> are completely interchangable, mere stylistic variants of writing the phoneme, and even texts written by a single scribe will often have the same word written with both letters. some older MSS will use a plain <d> for the voiced sound (and a plain <b> for the voiced /f/). i've seen at least one MS that used <th>. in Latinized versions of OE names, it's not uncommon to see <th> or <d>, but Classical OE spelling didn't generally distinguish the voiced or unvoiced.
even in Contemporary OE, there are very few minimal pairs between /þ/ and /ð/ ("thigh" vs. "thy" comes to mind, but not much else apart from rare noun/verb combos like "loath" vs. "loathe"). it could be argued that we don't really need both, but the (surely obvious by now) pedant in me desires both. saying "boð" rather than "boþ" doesn't strictly change the meaning, and is not likely to cause confusion, but it sure sounds off!
Well, we get those two letters "for free" anyway, and the phonemes are distinct in modern English, so let's use them!
"ð" also has the nice property of being sufficiently similar in looks to "d" that someone reading it and not familiar with the letter might pronounce it as "d" and thereby make a decent approximation. Unfortunately that's not the case with "þ" though. If we ignored the historical letters and instead just picked something reasonably sensible, I'd probably go with "đ" and "ŧ" for this reason.
i've had people mistake "ð" for an "o" with a diacritic, but presumably that's a typeface problem. whatever we choose, i insist that we drop the names and call them "eþþ" and "ðee", by analogy with our other fricatives ;)
even in Contemporary OE, there are very few minimal pairs between /þ/ and /ð/ ("thigh" vs. "thy" comes to mind, but not much else apart from rare noun/verb combos like "loath" vs. "loathe"). it could be argued that we don't really need both, but the (surely obvious by now) pedant in me desires both. saying "boð" rather than "boþ" doesn't strictly change the meaning, and is not likely to cause confusion, but it sure sounds off!