Exhaust sounds to be quite the interesting option. While this certainly does warrant some trial, it sounds as it exhaust would be the same thing as a second grenade as well as UPGRADE!!!, which already fill their roles adequately to my experience. And while exhaust certainly would help with chasing where grenade and UPGRADE!!! wouldn't, the role of chasing I feel should be left to chasers rather than a pesky Heimer. As such, exhaust would certainly hold viability, but from first examination seems to be quite situational.
Clarity, on the other hand, is always useful when Heimer does not have the golem buff, which cannot always be counted upon. Items, I feel, do not outlast the longest of sieges, and mana regen runes would help, but that would be taking away valuable slots for the all-important magic penetration.
And of course, my guide is not one that says clarity is needed, but rather the summoner spell to replace if one favours escape utility.
All-in-all, exhaust sounds to be a summoner spell with areas of superiority just as any other, but from first scrutiny, I still prefer clarity for its support in sieging, the role most substantial to Heimerdinger.