I personally almost never take HoL as Oak, but my reasons are largely due to my playstyle, and I'm sure some people justify needing to have it. Here's why I don't...
1.) I love, love, love priest minions. They up my survivability a ton and also my longevity on the field. However, consistantly using them with HoL is a pain, since their heal interrupts it. I value the priests more than HoL most of the time, so they usually keep me from getting it.
2.)Early to mid game, I have no room for it. My slots are taken up by Wand of Speed, TP scrolls, and flag locks. So until we hit war rank 10, I can't even begin to justify fitting the HoL in there.
3.)My build does not incorporate maxed shield time. I typically pick up rank 1 shield at level 3 and then never touch it again. I've gotten really good at using shield as a big attack is coming at me, and thats really all i need it for. Putting those points elsewhere allow me to pump up my damage substantially. Don't get me wrong, high rank shield is sweet as hell, but it's just not my style.
4.)I prefer to get stronger gear/cit upgrades. Why get a HoL if you can make yourself strong enough not to need it? Between priests and divine justice, I find that i can regenerate fairly successfully without it. Yes, I will go OOM sometimes, and that does suck, but I find that being that much more effective while I have mana is worth it. Plus, cit upgrades are always helpful.
5.)Certain opponents/situations make HoL nearly impossible to use. I've been in matches where I've been fighting trebuchet Rooks with Angels and it is damned impossible to get that thing to stay on very long. Kinda makes the thing a poor investment, and I find situations like this come up far too much for my liking.
Also, to answer your question specifically: I personally adore Mageslayer. If you're doing well, it's a relatively cheap upgrade, I also find that attack speed is super crucial to Oak, mainly because Surge of Faith makes it incredibly easy to get to absurd levels of awesome. Oak has plenty of skills at his disposal to up his weapon damage, so this is really the only time I'll ever buy an item to directly up my auto attack: the attack speed is what really does it for me. Plus, it makes you a pushing machine, able to take down towers at a stupid rate. However, it IS roughly 4k more than a HoL, so there would be certain situations where I wouldn't want to get it.